Glad to hear of a plan in action.
Just my 2 cents here but I would go with the larger tank, Its not gonna take much time and fuel for the larger and it will offer alot more real estate for good eats, maybe even a slide out shelf.
My scrap is going for .30 a lb making the small tank cost 87 bucks, and the 500 would be about 150.00. I would think the 50 bucks for a 500 is an awesome deal. I bet the 500 is a much thicker wall tank as well.
I have been wanting an enclosed trailer for awhile now but after further research I really can't afford that until the all mighty child support is over. Gotta love that. But since I collect 1 1/4" square tube from work by the 100's of feet. I am planning a switch over for my RF from the trailer its on, too a heavy tandem trailer. I really only need a 7000 lb tandem but I can get a 14000 lb'er for less. Do I need it that heavy? No. Will I be able to load it with no regard as to which side I put more weight on? Yes, to a point anyways. My point is, I always try to buy or build heavier then I think I will ever need. Heck if I was to run across one of those way cool off road golf carts, I would have to haul that too.