Roger, after you've given so much of yourself for the rest of us on the board, I couldn't resist the oppotunity offer some thoughts for you on this.
While I can't speak to the Selway, (it's next on my list and I'm sure it will be great), I have to say the Middle Fork of the Salmon is a world class trip. As many have said, there are plenty of rapids to keep the thrills high, the scenery, (despite some expansive, but interesting fire areas) is tremendous, the hot springs are a delight, the pictograph sites are moving, the wildlife is thrilling, but let me let you in on the best kept non-secret about the Middle Fork, especially since you said you and the wife are going.
The trip from Boundry creek is typically a late spring through mid-August starting point, obviously depending on the snowpack/melt. After that you will probably have to fly into Indain creek for the run down from there. Now thats not all bad mind you, as the river is still GREAT, and the scenery flying in is worth the price of admission. The best part of starting up high is the longer trip, (as stated above), through some wonderful landscapes. The secret....is that the fishing is damn near every cast to where you think there should be a fish, there is a fish that will strike. That means you can wear your arm out if you like, your wife can fish as little or as much as she wants, and there's still a ton of stuff to do amongst the aforementioned attractions.
I'd strongly recommend using an outfitter, (I was very impressed with Solitude River Adventures) that accomodates rafting AND fly fishing. They have rafts that are set up to accomodate fishing, relaxing and enjoying the scenery, cl IV rapids, etc. The owner is a hoot, (one leg and he's run the river thousands of times) and nearly all his guides are seasoned fly fisherman who understand that some people like to fish all day, and some like to to other things. They will cater to you and your wifes desires, and every dinner is like eating at a 5 star resturaunt, except your're surrounded by great people in a great setting. Depending on flows, some of his guides may also run the river with dory's. You and your wife are free to stay with the same raft or switch off as you get to know others on the trip. Bottom line is that it is a great couples trip.
As a point of reference, I don't think you can go wrong with any of the rivers you suggested. Hells canyon is very cool but I'd recommend a shorter trip on a jet boat, (they're all over the river and it's a different type of experience). The main Salmon gets tons of traffic so you'll have lots of company. Even though the middle fork gets a lot of people, it is very well managed, (permit only during season) so you don't feel crowded at all.
I could go on about the middle fork so if you want to chat more just drop me a PM.
Jeff
