Well, of course, I am biased, but come to Ecuador!
Ecuador is great in late fall and early winter. It has a TON of variety of rivers in a small area. There are countless Class III to V rivers within a 100 km radius, so whatever your style, you'll be happy with Ecuador.
As KSC stated, it is definitely more "exotic" than Costa Rica if you like that sort of thing. The people are super friendly, the climate is perfect (not too hot, not to cold) and the rivers are world class.
So...you really can't go wrong.
If you want a guided trip, you should, of course, check us out--www.smallworldadventures.com
But if you plan to go on your own and you just want more beta, I'm happy to help.
Just shoot me an email at firstname.lastname@example.org