If you want the best shots then bring what the pro's bring. All the pro's who change for their shots bring their standard land cameras but pack them in a pelican AND either put them in their drybox or have them in drybag for the rapids. Yes it is a bit bulky and a pain to lug it around but when your on a 22 day trip you will want to take a hell of a lot of shots and I know you want the best quality. You will be happy you brought it when you are taking awesome shots of me going through the big ones. Good luck with your decision. I know that either way I will have great pictures.
May the wind be always at your back,
May the sun shine warm upon your face.