I'm don't dislike FFXIII. It's not one of my favourite FFs but from where I stand, it's a decent enough addition to the line-up. In comparison to X though, I'd have to say I like X more. Mostly because of Blitzball, if I'm honest. The option of having a side-quest always available for you to do to get goodies when you didn't feel like moving on in the story or grinding is something I've always appreciated out of recent FFs (Triple Triad, Tetra Master, Blitzball, Marks). In FFXIII, you didn't get this until Chapter 11.

I find that the battle system is a big win for FFXIII though. I never liked FFX's battle system, though strategic, it felt slow. I much prefer the fast-paced, harder battles in FFXIII. In X, if you've got the wrong party member for the job, you just swap 'em in, but in XIII, you're made to have considered this beforehand and setup your Paradigms appropriately.

I consider the levelling up system in both games to be around the same in quality. Both start out very restrictive and open up later to let your character try out other classes. X had a little more room for customisation because you do pick up a few Lvl. 1-4 Spheres just as you progress through the story and it was always fun to mix-up Kimahri in replays of the game. But I still find both systems to be just as enjoyable.

^ Note that I say this having not ever tried out the Expert Sphere Grid before. So I might be missing out on something big here.