I've just recently played through FFX and FFX-2 back to back, and my opinion is that FFX-2 had the better battle system. I was warned that you would play a lot of dress up, but I never found that to be the case. It might be because my characters were vastly overpowered; I walked all over the world map every chapter and fought as many enemies as possible... heck, I didn't even go to the first hot spot of the game until I was level 20 (that took an extra trip or two down Mithen Highroad) and I finished with each character in the mid-60s. At any rate, Paine was a pure warrior, Rikku was a thief, and Yuna went back and forth between White Mage and Gun Mage, but rarely in battle.
But, in the end, I liked Final Fantasy X so much more. Each area in FFX was new, you never really had to go anywhere twice (and you really couldn't, either, until the end), whereas FFX-2 had mostly the same areas as FFX with subtle improvements, but you had access to everywhere at the beginning of the game. And the story of FFX was more engaging. Say what you will about the linearity of the game, but I preferred it since it meant that the story unfolded with every place you went and with everything that you did. FFX-2's system of hinting at some story here and teasing at some plotline there was a bit lacking.
I am also more than a bit disappointed that the normal ending for FFX-2 didn't bring real closure to the Yuna-Tidus story arch. It would have been nice if at least a tease had been thrown in. After spending so many hours with FFX and then following it up with FFX-2, I thought I deserved to see a happier outcome for the main protagonists of the series. But I didn't get it, and I just don't have the time to give it another run right now. Perhaps down the line.
Ultimately, give me FFX over FFX-2 any day of the week, but I still found FFX-2 highly enjoyable.