A Wii FFXIII would've hit market years earlier, been significantly less costly to make, and likely sold similarly in the end imo (maybe a bit better in Japan, maybe a bit worse elsewhere). It also would've resulted in a far different, and likely far less linear, game. It also could have been PS2 multiplatform, which would've raised it's market potential exponentially.
It also would've centralized the JRPG base on Wii earlier, which is a part of the reason you saw Tales doing so well on PS3 (in Japan) versus other consoles. Not that Tales makes for a great genre argument, it also bombed on DS while DQ hit record highs, and Graces f megabombed in the west compated to Vesperia 360 or even Symphonia DOTNW.
It's really moot now though, since SE forged ahead with likely the the worst managed and most overbudget major Japanese game this gen. And from the looks of things, they'll be repeating the same mistake with Agni's Philosophy/FFXV.