I know this is, like, my 100th KH thread, but the reason for that is I am currently playing both all of the way through. I was playing the first one & I am at the point where I am just defeating optional bosses & trying to get the ultima weapon (which is an absolute pain in the butt). Anyway, all the optional stuff is really boring so I'm starting the second game today & honestly I'm going to enjoy it a lot more.
The first game is.... more basic. Still great, but IMO the second game is vastly improved. The storyline is also improved. I'm aware the storyline of both games is really cheesy, but at least the second one is a bit more complex.