If it came down to FFXII vs. Zelda, Zelda >> FFXII.
TWW sold near 850,000 copies its first month during a slow month. The last FF game released (truely FF) hit 500,000 during a big month with very little competition among the platform itself. You can bet your ass both companies will be hyping their games, but Zelda will feature a much more badass main character that should draw in casuals and deserters of the series after TWW.
If it came down to KH vs. Zelda, Zelda > KH.
This one's tough. KH is really in touch with the kids, and a lot of teens-adults enjoy it too. If COM is very successful on the GBA, then these two might equal each other in sales, but Zelda, if it comes out in a month like November where Nintendo will be putting out Zelda Collector bundles (or maybe Zelda FSA bundles), then I think it will have the edge over KH.
If it was KH vs. FFXII vs. Zelda, Zelda > KH > FFXII.
FFXII and KH will be split between the Square fanbase, and Zelda should have an easy time taking the crown.
In any scenario, I can't see a way for Zelda to just fall flat on its face. Square Enix/Sony will be hyping up KH and FFXII, but for this gen, this new Zelda just may be their biggest title (it'd be hard to top SSBM's success), and it will be advertised to high hell. The only obstacle is Nintendo's advertising itself.
Xenon should kill all those, though. New game system with the momentum MS has had lately (and if it carries over into next year) = win.