Cardiff or Bristol by train

SS Great Britain
Clifton Suspension Bridge

I would choose Bristol as there's not a whole lot for tourists in Cardiff comparatively (tho Wales is beautiful most of the things you go to see are outside the major cities, unless you're wanting to see a rugby match). I agree with Nigel that SS Great Britain (steamship which used to ferry immigrants to Australia) and Clifton Suspension Bridge (amazing feat of engineering by Isambard Kingdom Brunel) are the best things to do in Bristol. It's also just a nice place to walk around and it's known for good food, craft beer, etc.- and it's a shorter train ride. Cardiff has its good points too but personally I would rather include that on a trip around Wales than as a day trip from Windsor.

Clifton Village in Bristol is full of lovely Georgian buildings, shops and cafes. It’s nice for a walk around and to take in the views of the suspension bridge. You can walk across the bridge to the visitor centre on the other side. Walk a bit further and you’ll come to Ashton Court. This is a large estate now owned by the city with country walks, views and a deer park. The house is not open but the stables have been turned into a cafe.

The Harbourside area is also a good place to explore. As well as the SS Great Britain there’s the MShed museum which has the Colston statue (the one that was pulled down and thrown in the docks in an act of protest) on display among other things. You can also find 2 art galleries down there: Spike Island and the Arnolfini.

In Brandon Hill the Colston Tower gives great views of the city. There’s the Georgian House museum nearby which shows how a wealthy Georgian merchant would have lived.

To reach either city from Windsor, you would first have to take a train from Windsor Central to Slough & then transfer again at Reading.
This is all very easy.

Both Cardiff & Bristol are well worth visiting but of the two, I would favour Cardiff. Bristol is rather more sprawling than Cardiff & although it has the newer Cabot Circus Mall, the older Broadmead shopping area is dreadful 1950’s early 60’s architecture. Cardiff on the other hand has modern malls (St.David’s Centre) but the older parts of the city centre are generally late 1800’s and therefore have attractive architecture - for the most part. The city also has several arcades dating from the late 1800’s which contain unique shops.

Bristol does have some areas of very attractive architecture - notably Queen Square (Georgian) and Corn Street and the hilly Clifton area has architecture consisting of limestone terraces - rather similar to Bath. The harbourside (old docks in middle of city) does not in my opinion compare with the waterfront at Cardiff Bay which has the modern Wales Millennium Centre (Opera House/ Concert Hall) plus Senedd (Welsh Parliament), Norwegian Church and many waterside restaurants overlooking the Bay. That said, I really enjoyed my visit to the SS Great Britain in Bristol and also saw nearby ‘Matthew’ = a replica of the ship that Cabot sailed from Bristol to the new world. I also climbed the Cabot Tower for a view across the city. Bristol Cathedral is also in the central area whilst the Cathedral in Cardiff is 3Km NW of the city centre - at Llandaff.

It is the 4 castles in and around Cardiff that really sway things in favour of that city.
https://www.cardiffcastle.com/the-castle/ is found in the centre and dates from Roman times. In the middle is a Norman keep (1087) but much of the castle dates from the 1800’s when it was owned by the Marquess of Bute. Bute Park - behind the castle is also worth seeing.
CAERPHILLY CASTLE is a massive medieval fortress just a 20 minute local train ride north from Cardiff. The town of Caerphilly is a dump in an ex mining area but the castle is everything that you would expect of an 1200’s fortress complete with moat. If you are visiting Caerphilly, you might like to break the journey at Lisvane station and walk for 5 minutes across the car park and into CEFN-ONN-PARK (free). In spring it is very colourful with azaleas, camellias, rhododendrons and bluebells all under the canopy of a temperate rain forest. https://www.outdoorcardiff.com/parks/parc-cefn-onn/
https://cadw.gov.wales/visit/places-to-visit/caerphilly-castle

CASTELL COCH stands on a hillside in north Cardiff. If you have a car - it is M4 junction 32 and you could easily visit this castle and Caerphilly Castle in one go - or taxi between the 2. If visiting by bus from Cardiff - you need Stagecoach 132 from Greyfriars Road and alight at Tongwynlais (about 25 minutes out). The walk from the bus stop to castle is about 17 minutes uphill.
https://cadw.gov.wales/visit/places-to-visit/castell-coch

ST.FAGANS CASTLE & MUSEUM OF WELSH LIFE is just to the west of the city. Cardiff Bus 32 - be sure to check schedule! The site is vast and needs at least half a day. You have grounds with buildings of significance brought from all over Wales and re-erected. The Castle is really a 1500’s Manor House. You will also find gardens near the castle. You will also find indoor galleries explaining about life in Wales through the ages - handy if it starts to rain. https://museum.wales/stfagans/about/

https://www.visitwales.com/destinations/south-wales/cardiff/top-things-see-and-do-cardiff-bay
THE NATIONAL MUSEUM OF WALES is found just N of the shopping area next to the City Hall. It has extensive art galleries and Geology of Wales etc.

Wow! Thank you for the recommendations. It seems I must rethink going to Bristol? More research….

Both cities are well worth a visit but marginally I would say that Bristol has the most interest. Both in my view have more than Bath to interest a visitor.

One is the National Capital of Wales with modern buildings at Government

Both have been working ports

One is much easier to get to because it is closer

Both have had some regeneration

The vibe and spirit is completely different between the two

One is and has been very gritty and the other less so.

Both are cities but Bristol is much larger.. Cardiff is about the population of Arlington Texas, Bristol is between 2 and 3 times larger at 3/4 of a million.

With so many things to do and see in and around Bristol it has more appeal to me personally.

Cardiff lets you check off another country.

Bristol has a diabolical on-way scheme in the centre. I would advise not diving in the centre of either. Parking in Bristol is no fun.

I understand the comments, but we had no trouble whatever driving and parking in Bristol. It's one of our favorite places. We stayed on Queen Square our last visit; there's a Premier Inn nearby.

From Windsor, you need to get to Reading to catch an express to either Bristol Temple Meads or Cardiff Central. Reading to Bristol is around 1 hour 10 minutes and Reading to Cardiff is around 1 hour 25 minutes - so nothing much in it really.

According to the Bristol City Council website, the population of the city in 2023 was 483,000.
In 2024, the population of Cardiff was 492,000 - so, nothing much in it really.

Also note that outside the city boundaries are other nearby places with significant populations. Penarth & Barry on the coast near Cardiff do not count towards the population of the city because they are in another county. North of Cardiff are heavily populated valleys once famous for coal mining. It is only when you go about 40 minutes drive north of the city that you enter the more sparsely populated farming country that makes up the bulk of Wales - though there is also attractive farming country around the city - especially the Vale of Glamorgan to the west. That said, Bristol also has attractive farming country near the city.

According to the Office of National Statistics, the population of Cardiff at the 2021 census was 362,400, up 4.7% from 2011.
So unless there has been a near 33% increase in three years there is a big data disconnect here.
As the entire population of Wales is only 3.1 million such an increase in three years would be highly newsworthy.I
But equally Nigel's figure of 750,000 for Bristol is over 250,000 rich according to the ONS.

If you go to Bristol, I think that for me the stand-out attraction is SS Great Britain, one of the best historic ship museums/attractions I’ve seen. https://www.ssgreatbritain.org/

Not that far away is the beautiful Queen Square, a wonderful Georgian Square.

Bristol Cathedral is worth a look too. All these are walkable between.

Clifton Suspension Bridge is another big highlight, but is really a bus or taxi ride away unless you enjoy very hilly walks. There are some lovely streets around Clifton that are very genteel Georgian in a Bath kind of way.

The larger population figure probably refers to the greater Bristol area. Parts of the city are technically in neighbouring counties. By this measure the city is larger than Cardiff.

You can conjure up all sorts of population statistics depending on what is being measured. Almost certainly the lower numbers are based on the populations or the two local authorities but it is equally correct geographically to include the greater urban areas, which will include other adjacent local authorities. For example quite a lot of the Bristol conurbation to the north east and east is in South Gloucestershire.

well when I was a boy (in Glos.) we used to think that Bristol was in Glos. (all the best things were), and it had been at one time but it had been some time since they had moved things around (about 600 years!!)

For what it's worth, I just spent a 5 days in Bristol, with a day trip to Cardiff, this past August. I loved Bristol! Amazing food, beautiful town, completely walkable (except to the suspension bridge--that area you need to take transit or a taxi to get to.) I have to admit, Cardiff for me was just meh. It had been a place I really wanted to see so I don't know if I had built it up in my mind but, while I'm glad I went, it was just okay.

Getting between Bristol and Cardiff is super easy by train so you can definitely stay in one and day trip it to the other.