you seem set on the mega. okay between the mega and the 7xl. both will be run on 3s for now. and looking at the site, the mega that you pointed out can only handle 3s lipo, not 4s. and im still not positive that i can get one
It handles 4S, if you look at that link it says 12 cells which equals to 14.4 and 4S is 14.8 volts. If you go with the 7XL make sure you get the best lipos you can find, because they will consume a lot of AMPs. The Mega will need less AMPs and will have more torque.