To me FF 12 felt like a chore and I never touched it again after finishing it at 120 hours. On the other hand I'm at 80 hours on FF13 and I plan to play many more. I've finished the plot, which I really liked, and now I'm doing all the sidequests and going for the platinum trophy.
12's plot felt like an afterthought due to its drawning out spacing between gameplay stages and the disconnect there was between the main characters and the main plot line. It didn't help matters that I was displeased with the battle and gambit systems.
13's plot on the other hand was always present, it was a focus (pardon the pun). Of course the linearity of the game helped immensely to make it feel more cohesive. But still, there was a direction and an intent and the game kept you updated on everything at all times with the datalog. Also, the battle system was fast and frantic, which I loved, and the paradigm system shone brightest when the battles were harder as you had to furiously switch between them.
All in all: 13 > 12.
