Each game (KH I and KH II) has their pros and cons.

KH I

What started it all for me was a TV commercial. KH I was the first game I purchased with my new PS2. I instantly fell in love with Hikari/Simple & Clean and the opening. What stuck with me was the plot. At the time I had not long ago read William Golding's Lord of the Flies and so the plot had reminded me of that. For a game with Disney in it, it proved to be surprisingly serious. The FF cameos and the Disney ones were for the most part well done (from what I remembered). The fighting mechanics were decent though they get improved upon in the sequel. I liked the spell selection, especially the fact that Graviga was your friend (I was sadden when it was removed). Overall the difficulty was consistent and good.

KH II

I hated the first two hours of the game. Okay, not so much hate but bored the hell out of me. Once you got to fight, I noticed that the battle mechanics were better (as well as the camera!). Maybe I am lazy but I liked the fact that you could rush towards enemies. The addition of the Reaction button was interesting. It made fighting hords of heartless a lot faster. Sometimes you just want to farm for some synthesis items. Sadly the gameplay really went down in difficulty. Proud Mode offered few challenging fights. Oh did I mention that I am in love with Passion? Awesome song. The plot remained pretty good but lacked the depth of the previous one and seemed to be a sequel with prequel information (i.e. you basically found out all about Ansem and such and part of the events that led up to KH I). Though I think the whole heart, body and soul distinction was interestingly developed further. I wonder if a "soulless" or "no souls" will be next. :P Disney in this game was both good and bad. Atlantica? WTF? Seriously. W.T.F. Timeless River was nifty as was the fact that you finally get to go to Disney Castle (anyone ever noticed that in KH I, Queen Minnie's head was monstrously disproportionate?). One thing nice about KH II is that if I want I can pick it up and just play for a little while. Both games had extra content which was nice, as well as they advanced the gummi system in KH II. The sucky part is the whole Custom Gummi creator got too confusing for me in II. :/