The American one appeals to me in a sort of 'scrapyard tank' way, but I like the Japanese one better. After all, who doesn't love sleek curves and a full HUD?
Yeah, I liked the gritty "cobbled together" look in Megabots' build. Something appealing about making new tech with what you have available. Japanese mech looks like a Japanese mech straight outta anime. I actually like the American design more, it's got this whole "function over form" approach to it, while I feel the Japanese mech was going for an "anime" looking mech. I love the Japanese mech's control scheme though, that was pretty clever.
That said, I think both models are quite impractical.
If you want a practical armoured combat vehicle, get a tank. Mechs/mechas/giant robots/exoskeletons are designed to be CARPIN' BADASS, not actually practical in any shape or form.
This style of thinking could actually be detrimental to the future business of GIANT ROBOTS. Why make GIANT ROBOTS when you could make a tank that does everything a GIANT ROBOT could but in a more practical and efficient manner? It wouldn't be a practical business model or investment. GIANT ROBOTS would need to have a clear reason to stand out and be made besides just looking cool.
I think one positive the mechs have going for them are requiring less staffing than a tank. Tanks take several people to drive it and manage everything in it, as far as I know. Both models of mech, the American and Japanese, require just one (Japan) or two (USA) people. Maybe the tanks would be better at tanking while mechs could be made as "shock troopers" loaded with all manner of heavy weaponry.