While I like the fact that the Japanese cover has a lot of color and is livelier, the one thing that instantly stands out is the amount of cleavage Maria is sporting. I understand that sex sells, but it's a little too obvious that Tomoe and Maria are fanservicing up the Japanese cover a LOT.
The US version has a more minimal design to it that I think fits with the "medical drama" theme, but I have to admit the rainbow of colors on the Japanese version is very spirited.
Personally I think the reversible cover idea is the best of both worlds. That way you can a serious US-style cover with the paneled portraits for store display while giving the purist (or perhaps puerile) fans the bright, *ahem* eye-catching cover they desire.