I picked up both at Black Friday for super cheap but haven't played them yet. I usually play more in the summer when there's no hockey or football to take up so much bloody time. I've wanted to get into the series for a long time.
My non-spoilery opinions:
KH1 is the best action RPG I’ve played. The bosses are great, the fighting system is simple but nuanced, you can hack and slash your way through it by grinding levels but the magic and combos have high reward. The story is also so fun and enjoyable, especially if you are even a casual Disney fan with even an ounce of nostalgia in your body.
KH2 is inferior (unpopular opinion) for two reasons:
1) the story picks up after Chains of Memories, which is one of the worst games ever made (their GBA game). It’s also convoluted and impossible to follow unless you know what happens (and difficult even then). It’s less about Disney and more about the Final Fantasy-esque world they’ve created, which isn’t as fun.
2) the gameplay is way less strategic and way more button mash-y. The bosses in KH1 are all the classic “figure out their patterns, find their weak spot, maybe a trick or two (hop onto Cerberus’s back, you’re welcome), and battle them man to man.” No real gimmicks, you’re the same player every time and have to figure them out. KH2 introduces an “action button”; any time a special move is available a big shiny green triangle button appears in the bottom corner and when you press it you do a fancy, world-specific move. This, unfortunately, becomes the way you beat every single boss; just survive long enough to do the action several times and you win.
That’s not to say KH2 isn’t worth playing (and many appreciate the expanded battle options in the second one, I prefer the simplicity of the first). I still play through both when I do go back (every 2-4 years, thankfully they keep re-releasing them). I just have a clear favorite. I find I’m in the minority though, most seem to prefer KH2.