Interesting question. A new numbered Kingdom Hearts is a big deal considering the almost 10 year gap there will have been by the time Kingdom Hearts III releases, so the hype from that could boost it's numbers. But Final Fantasy XV is an interesting situation. The Final Fantasy brand (whether one likes it or not) was damaged with recent releases, which could effect XV's chances, but because XV is actually the heavily anticipated Versus XIII, can help it (not to mention it separates itself from XIII a lot just from it's style and gameplay). Whether any of this actually has any bearing on the sales is unknown, but I do think XV will come out on top. There's just more hype for it at this point and it has been talked about for much longer just because of it's long development cycle.
At this point it's FAR too early to be making predictions for these games, but if I forced myself to, I'd say Final Fantasy XV will reach 6.15 million and Kingdom Hearts III will sit at 4.95 million (until they start revealing some HUGE Disney cameos which will convince me to up my prediction for Kingdom Hearts III).
