In America, Kingdom Hearts is bigger than FF. It is the only region in which this is true. KH is the best selling console JRPG in the region (besting even FFVII), and KH2 outsold FFX-2 / FFXII. KH:CoM sold tons as well. Also see:
KH 358/2 (will outdo both without trouble)
edit: can't get KH 358/2 links to work somehow
Compare that to, say, Japan, where they probably don't care for Disney as much:
I attribute much of this to the "Disney factor." On the PS2, GBA, and now DS, America has shown its KH fanbase is larger than FF's.
KH: BBS would do better on the Wii for a few reasons, one being the above. The other reason is PSP software being a bit weak in the west. Either way, I'm not sure it's worth a port at this point.
@ posts above: I think a decent test is if KH on the PSP doesn't outsell its FF's (Crisis Core and especially Disidia). I imagine there are more Square Enix fans than Disney on the PSP (though I could be wrong).
