Well Barley, as much as you hate it I'm going with the 50qt.
I'm going to insulate as others have said with the "great stuff". There isn't a spigot hole, so I'll have to drill one. There is a small channel(low spot) on one side as if there should be a spigot, so I'll put it there. I like the shape and size of the 50qt. it just seems to work for the setup I have in mind. Also without the wheel well, I won't have the 4" of dead space in the bottom of the tun. I'm going to fabricate a false bottom for it.
I was thinking about what you said about the lid losing it's seal after a while. I think if I have that problem I'll put a strip of weather proofing around the rim and a latch or piece of velcro( or something of the like) to make a nice tight seal.
I do like the little lip around the inside about 2 inches down from the rim...perfect to hang a sparge arm from. I'm not sure I like the sparge mounted to the underside of the lid because i can't see a way to monitor the flow effectively. It won't flow the same while the lid is up.
I
do appreciate your input and your opinion Barley. Thank you for your help.