If you HAD to choose between these two ...

Visited all these places, but if I had to choose, North Wales would win hands down. If you decide to make your destination Wales, I would cover that part of your journey from Chester and then head off to the Cotswolds. I presume you are flying out of London?

Personally, I would cover both of these areas and drop the Cotswolds.

Is this a poll?

Salisbury and Winchester. (Throw in Jane Austen country as a bonus)

As they say, there is more than one way to skin a cat.

Could you double back to Salisbury after your morning at Highclere on the 25th?
Drop the car and train to London on 26th, perhaps dropping Windsor Castle.

If you do find yourself in Winchester the fly fishing on the Itchen and Tyne is amazing; as is the The Wykeham Arms hotel tucked away above a pub in the center of town. Great room, great people. We really enjoy Winchester.

Thanks for all the replies and suggestions. I hear you all and appreciate the advice. I suppose we could drop one day in the Cotswolds, just spending one night, and just do a drive thru on the way to Bath - this is what all the tours of the Cotswolds do anyway. Then that one day could be spent between Conwy and Caernarfon - would this give us enough time to visit both on the same day? If you see my other post, "revamped itinerary" we are including Stonehenge, Avebury, Devizes, Kennett Upon Avon Locks, etc. on our way to Highclere. Stonehenge was a must for hubby!

I would also skip the Cotswolds and do both options, but if you must choose, I would choose Salisbury and Winchester because I'm a cathedral nut. The castles in northern Wales are very nice and I enjoyed our time there (we had 5-6 nights there).

This is getting interesting! I've read so much about the Cotswolds, how quaint, beautiful, unique, etc., and I really appreciate such honesty regarding spending time there. I have heard a huge negative to the Cotswolds is the congestion in the busy summer months. Would N. Wales be quieter? Think I'm leaning towards cutting a day and night from the Cotswolds to squeeze in time in N. Wales. Our only concern there was we won't have time to really explore deeply, just get a taste this time around. I expect it would still be worth it?

Another consideration would be how easy your options are to get to on another trip. London Heathrow is an easy airport to reach from N. America. If you were making a quick trip, you could easily combine Salisbury/Winchester with London and not even rent a car. If you did rent a car, the Cotswolds are an easy drive from London. But getting to northern Wales from London takes more time and effort and you really need to get a car to get around up there. If you were to have a stopover at Heathrow with a handful of days, Winchester/Salisbury or the Cotswolds would be very convenient.

Maybe something like this would slot in ...

Sat., 16th. To Scottish Borders, Hadrian's Wall, Once Brewed. Keswick O/N.
Sun., 17th. Day in Lake District. Keswick O/N.
Mon., 18th. Keswick to Chester. Chester O/N.
Tues., 19th. Excursion to Liverpool. Chester O/N.
Wed., 20th. Excursion to North Wales. Chester O/N.
Thurs., 21st. Chester to Cotswolds. Cotswolds O/N.
Fri., 22nd. Day in the Cotswolds. On to Bath O/N.
Sat., 23rd. Day in Bath. On to Salisbury O/N.
Sun., 24th. Stonehenge at first opening - 9:00 am. Salisbury to Winchester. Winchester O/N.

I did not think Salisbury as a city was that great (we stayed two nights, arriving in early evening), but it's cathedral is absolutely spectacular. If you make it there, take the special tour of the upstairs. I think it was around $15 but well worth it. Along with Chartes, this is my favorite cathedral of many, many that my wife and I have visited. The city museum is fairly interesting in Salisbury as well.

Hey David in Brisbane - that's a brilliant suggestion! Thank you! Sometimes it definitely makes sense to have other sets of eyes look at an itinerary. I'm sure we can wing this route and still get enough of the Cotswolds area in for a look see. If we like what we see, we can see more on our next trip. I was really hesitant to skip Salisbury and Winchester - seemed like they were on the way to London anyway. Also day of castles in N. Wales seems like a nice day out. We did do a day of castles out of Copenhagen via train a few years ago - it was a long, long day, but soooo worth it! Even did Tivoli that night upon return - we were exhausted but exhiliarated. Thanks again.

If you wanted to gain a day in north Wales, I would skip the day in Liverpool.

From Chester to Conway/Conwy and Caernarfon - you have the A55 expressway as far as Bangor with just a short bit of slower road to Caernarfon. However, it is the inland routes through the mountains that are really scenic - but much slower going.

Someone suggested a tour inside Salisbury Cathedral. It is guided tours like that which eat up time. If you are pushed for time, see Salisbury Cathedral from the outside. It really is a beautiful sight.

Yes, I think it would work well for you. It is scarily similar to the exact routing we took in 2004.
We came from Glasgow and had 2 nights in Keswick, followed by x3 one week cottage bookings in Chester, Cheltenham, and Cornwall. From Chester it is a long day into North Wales, but we did Conwy in the morning, Beaumaris+Anglesey at lunchtime, then Caernarfon in the afternoon. Similarly Liverpool is easy from Chester across the Mersey at Birkenhead.

Thanks James, but a resounding No! I cannot go all the way to England and not see Liverpool - you see, I have been a huge Beatles' fan since I was 12 (and that was longer ago than I care to admit!). Hubby is humouring me, as I am humouring him with Stonehenge. Then on the other hand, he is also humouring me with Highclere.

If in Salisbury, don't neglect going to 'Arundells' if it is open. This was the home of Edward Heath and is full of various stuff he collected through his political career. Unlike most places of its type you are encouraged to sit in the chairs. There is a long history to the house itself. As it is in the Cathedral Close it isn't hard to pull it in although pre-booking is probably a good idea.

I'm not wild about the idea of north Wales as a day trip -- it seems way to rushed to me -- but if you don't mind that kind of pace, David's idea is a good way for you to see everything you want to see. Liverpool works well as a day trip, by the way, and if you are a Beatles fan, I think you'll really enjoy it. I've only spent an afternoon in the Cotswolds, but I loved it. I wished I could have spent more, and I'm still hoping to get back for a longer visit. I hope you have a great trip!

So it looks like we can have our cake and eat it too. The only change from David's suggestion is rather than backtracking to Chester after a day of castles, we'll head southeast from Caernarfon to Llangollen and south from there. This way we can get our feet wet in the Snowdonia Mountains on our way through Llanberis and Betwsy-Coed, possibly staying near Llangollen for the night. Next day, carrying on to Ludlow, Shrewsbury; seethe Pontcysyllte Aqueduct, Iron Bridge, Stokesay Castle, etc. on our way to the Cotswolds. This cuts down our time in the Cotswolds, but that's OK. Like another poster said, the Cotswolds are a short distance from London for our next trip to the UK!

Another good idea lorierae.