Japan. No question. SMB2US is probably my least favorite mainstream Mario game up until, well, N64. SMB2J is what I was hoping SMB could be, difficulty-wise, and a few of the level designs are absolutely brilliant. (I'm talking about you, 8-2.)
I mean, ya, SMB2US had a few good levels (3-3 comes to mind), but it just seems... I don't know ... not Mario-esque? I mean, ya, Nintendo could've picked a MUCH worse game to sprite-swap than Doki Doki Panic, but still...