I have an AIRE Lynx II. It is amazing how stable and fun they are. I got my son into the sport by taking him on rivers. Now my wife uses it to take the dog on rivers while my son and I have our own boats.
Ditto all above. The Lynx II is rock solid, high quality, and pricey. I couldn't find it discounted. The air floor is the way to go. The foam floor is for commercial outfitters that use trailers and keep their boats inflated.
I rented a Tomcat II, also by AIRE, last summer. It's a new cheaper boat the same size as the Lynx II, but I didn't see any significant difference between the $600 Tomcat II and my $1200 Lynx II.
I've seen the NRS, Sevylor boats and some others. I think the AIRE is definitely better and probably worth the extra money if you stay with it. You can rent the Lynx II and the Tomcat II from Alpenglow Sports in Golden to try them out. Other places will have the cheaper ones.
The easy, smooth runs these boats are great on are Chutes, Pumphouse, Shoshone, probably Westwater, and a bunch of others.
One suggestion: take safety and gear seriously when you get started. Do some easy test runs. You'll need gear to keep your legs warm since they get splashed a lot.
I found this small mail order store to have good equipment and good information: http://www.theboatpeople.com/index.html