I shall agree with Vergo and say FFX is the one I think is better out of the two.
Unlike FFXII, FFX kept my attention and had me wanting to play it and level up my characters. The story in FFX was a contributing factor to why I wanted to play it so much and think it's better. I loved the story in FFX. It caught my attention and held it throughout the whole entire game; FFXII didn't. I played a little bit into the game (can't remember where I left off, it's been a while) and got bored of it. The story didn't keep my attention and the only reason why I was still playing it was because I wanted to play something and that was only thing I had at the time. The fighting style is another reason why I think FFX is better. In FFX, I liked the Random battles you would have that would take you to another screen in order to fight the enemies you have encountered; FFXII didn't have that. You fought the monsters while still walking around; it got rid of the different screen you saw in order to fight them. The characters were another thing. In FFX, the main characters they had seemed interesting and I wanted to get to know them better as the story progressed. In FFXII, I didn't get that same feeling. I felt that how I had seen the main characters when I first met them was all that they were going to be. It seemed to me that they didn't really have any character depth. Although I could be wrong on this impression, since I've never beaten the game myself, but I have seen someone else play and beat it.
But yeah, there's probably a few other things that I could mention, but at the moment I can't think of them. I'm not completely hating on FFXII though, I mean, it was a good game, but in terms of it being better than FFX, I'll have to disagree on that by saying that in my opinion, FFX is better.