There should be a few Springs on the Thompson as well as I'd imagine a crapload of Sockeye moving through the river (I don't know if you'd want to fish for them though, or if it's even legal/ethical but they should something spectacular to see), that's if you want to know what is there fishing wise, although Dana mentioned he is interested in the casting aspect more so.
My only concern with the Thompson would be places to stay (camping/accomodations, where would they be? that is if there are a lot of people, is there enough room at the various local hotels and what not as well as campsites?etc.) and it might be advisable for everyone to have some sort of stream cleat for wading and a staff as well.
I think the Squamish might be a bit more of a hospitable casting/clave destination. More places to cast/choose from (as in spots), closer to a major centre, more things in the town of Squamish to do (for the wife or whatever if this is going to be an overnight or more than one day thing for anyone travelling), see, places to stay, eat, etc. The Squamish is a big river which runs with glacial run off for most of the Summer which makes it quite silty. There are a few Dollies and Springs around in August on the Squamish, but you have to consider yourself hardcore to go after them and find them in strong numbers with the two hander. This might make the casting more the focus than not although it'd be neat to catch the Odd Spring if they are there.
The one thing that is sad about the Squamish is it's Steelhead are in trouble. It's one of the most awesome Spey fishing Rivers in North America. NOt because it has lots of fish, but because the runs cater so well to fly fishing, with lots and lots of nice consistent runs with good depth and flow. If it had lots of STeelhead it would be a serious Steelheading desitinatin (keep in mind it only has true Spring Steelhead in small numbers) It's also a bit bigger of a river, not as large as the Thompson, but big enough that you have your work cut out for you with the Two hander and your maximum casts (even Steve Choate's maximum casts!).
What do you think Dana?