Persona 5. It is going to be a much better game than FFXV, I'm sure of it.
I'm sure both will be fine games by their own merits. I don't understand why this has to be a contest. Again, it would be best for both games to be released at different times in order to maximize both game's sale potential. There is no need to pettily drive by post X game is better because blah blah.
My biggest fear with P5 will be that it's just P3/P4 with better dungeons, but if you are telling me all you want out of P5 is an HD P3/P4 with better dungeons, then I don't think I would call it a better game. 8 years spent waiting just to play a better looking modern era Persona game.
FFXV, by the very nature of the Final Fantasy series will be divisive, some will claim it's the greatest game, the greatest FF game, the worst game, or the worst FF game. At least it's shaking things up with the mainline series.