the weird thing is, i have spent more time on ffx (because of familiarity) yet overall i think that persona 4 is the better game.like what you guys said ffx is more well known (first came up during early life cycle of ps2) as compared to P4 (swan song of ps2).
my personal opinion about the two games (which are both awesome in its own way)
P4 Social Link System
FFX Sphere Grid System
P4 has persona compendium to complete
FFX monster arena to complete
P4 is a high school life - murder mystery - identity crisis - save the world story
FFX story about death - love - dreaming - save the world story
P4 has 3d and anime style graphics
FFX fully 3d rendered
P4 made live concerts for its soundtrack
FFX audio cds
P4 tons of mini-games most of them are directly related to your progression
FFX BLITZBALL
P4 Margaret & The Reaper ( because you have to fight them 1v1 and no cheap way to beat them)
FFX Dark Aeons, Monster Arena, Penance ( Zanmato kind of kills the fun)
P4 story progression can be altered
FFX linear