What stat to stack for mages.

INT would be better to stack, yes. Aside from providing 1 magic, it also has another small part to play in the damage formula.

However, MW does not increase stats provided by gear, iirc.

This, though I'm not "iircing" because MW doesn't. I thought it was 100% confident common knowledge that MW only boosts off the base stats.

Anywho, OT: like everyone said, 1 INT > 1 MATK, so typically Magicians stack INT as much as they can, though obviously we invest in MATK where only MATK exists (or where stacking it would be better than INT). Like Phoenix Wright said, it also plays another small factor in the magic damage formula. Sorry to have your logical conclusions fall short thanks to a horribly designed magic damage formula -_-.