Get a 10lb tank. Easier to pack, more versatile, and provides more redundancy for propane failures. Small/short trip? Just bring that and some backup cylinders. More people, longer? Have someone else bring theirs too. At least for starting out. Wait to get the bigger tank until you find you wish you had it a couple of times. It's so tempting to get more/bigger but as I raft more I prefer to take less and less stuff if possible.
I am a river, babe - I've got plenty of time, I don't know where I'm going, I'm just following the lines..... - "We are water" by Shaye