I think I would use the 50 gallon, taking the time to remove the rim first. It has such great dimensions, 36 x 18 x 18 that it has to be an ideal "high water" riparium. But, if the rim is still there it would look awful, in my opinion. Just handling that size tank to remove the rim would scare me away from even trying. It looks like several hours of work, plus the time to thoroughly clean away the silicone deposits. (However, I'm just getting too old to tackle that kind of project.)
Really, well that will mean that I will have to transfer the fish and everything else twice more. The 50 is the only space I have right now, so I'll have to go get the 40 that's at my brother's house, move everything into that tank, de-rim the 50, then transfer everything back again. I had thought that the 40 would be a good shape because it is wider and deeper than it is tall, and could thus emphasize the view through the water's surface. I suppose the 50 will do that too if I raise the water up high enough. Do you think it will be safe if I bring the water up to about five or six inches from the top? I have wondered about some kind of reinforcement, such as small aluminum brackets glued around each top corner.
I suppose that taking that rim off will be about like it was for my 65, which took all day long.