I use the 20# model and with my bad back its not too bad to lug around but it def isn't light. I would think a full 50# tank would be pretty hard for one person to move, unless you employ a hand truck or something similar, then it would be a piece of cake. And then there is the size difference, do you have appropriate transportation for a big tank? If you can do it easily, go with a 50# tank. If not, make your life easier and get (2) 20# tanks and rotate them.
Good luck and keep it green.