Mario Clash has two versions. Post differences between them on this thread.

Although they seem to be the exact same, they are actually different, and this difference is felt in the later levels. American is easier to complete because enemies move slower. This gives American version an advantage in survivability, and possibly also in runs as well. Be sure to get both, because you never know if the faster enemies in Japanese could actually provide the benefit in getting faster times. Even if Japanese has no advantages whatsoever, you might want to get it anyway for training weights.