Yea the more I think about it the more I think ill do the under top tube tank. I have a large frame MTB, 21" size i beleive and I may as well take advantage of all the space i have between motor and frame plus I like the better weight distribution towards the front. The only negative is the size of the inlet, not sure a gas pump nozzle will fit it so may be forced to carry a small funnel if you ever fuel at gas stations, I know I like to fuel at home where i can mix the fuel in a can but sometimes you have no choice..
Problem is like you said there all out of stock!