Oxford or Windsor Castle?

If it were longer than a day trip, I’d vote for Oxford. But that takes a bit of time and Oxford is worth more than one day. So I would go with Windsor. It’s easier to get to and you can easily see a lot in that one day’s time.

Windsor Castle is decorated for Christmas now so should be particularly nice.

The forecast is that next week should pivot right back to pretty mild again, after a storm clears through over the weekend. So back out of winter to early fall.

The castle is closed on Tuesday and Wednesday, but apart from that I would favour Windsor. If you go at the weekend there are still short 40 minute boat trips running on the river as well.

There is also an open air ice rink open in town for the Christmas season.

Definitely Windsor Castle for a day trip. No need for a tour - trains leave from Paddington (to Windsor & Eaton Central via Slough) or Waterloo (direct to Windsor and Eton Riverside).

Just note the castle is closed to the public on Tuesdays and Wednesdays, and on Sundays St George's Chapel is open only for worship services, no tourist visits permitted. It does also close frequently for state occasions, and is closed next Thursday as well.

As noted above, there's a light show currently on the round tower - if you're there after dark it looks good.

I'll just add that if you do Windsor you can use your contactless CC to tap in and out on this line, so no need to book ahead. There are big signs to remind you to tap out at the Windsor and Eton Central station.

https://www.gwr.com/your-tickets/smart-tickets/contactless-payments

Re contactless card payment for Windsor, this is true for the route from Paddington to Windsor & Eton Central (as noted). Worth mentioning you can't use contactless on the SWR route from Waterloo to Windsor & Eton Riverside. It is due to be introduced but this has been delayed.

You don't have to book ahead anyway. You can also buy paper tickets on either route at the same price, on the day at Paddington or Waterloo respectively. But from the machines or the clerks.

This is all good info. Is a self-guided audio tour sufficient for sights in Windsor/Windsor Castle?

Leaving from Paddington, is there a transfer from to train getting to Windsor, or is it non-stop?

Paddington to Slough then change to the little one car train which goes around to the left to Windsor & Eton Central. An easy change.

And I'll say yes to the audio tour being more than enough for the Castle! I usually bring my own ear buds with a round plug-in and I'm pretty sure I used them there instead of the ones that were furnished.

Depending on the day, there may be an Evensong service at Saint George's chapel if you stay late enough to catch it. Everyone is ushered out of the castle and the folks waiting for Evensong are told where to stand at the castle exit, then let in at the appropriate time shortly before 5:15 when the service starts.

https://www.choralevensong.org/uk/st-georges-chapel-windsor-castle-116.php

When you get off the train in Slough, about half the train or more will be getting off with you and will head to the little short train that just goes back and forth to Windsor. There are always folks to ask if you are not sure but that track is well-signed. It's just a journey of about 6 minutes between the 2 so also no problem if you have to stand.

What you see in the town of Windsor (or is it a city??) will probably depend on your weather.

Windsor is a town.