Here's my take on things.
KOTOR 1 - A more fun game, more "light", simple story, adventure-filled, space-opera-epic game with a few "good" characters here and there.
KOTOR 2 - Gameplay wasn't as entertaining or fun. But man that story, the writing is heads and shoulders above KOTOR 1. I mean, it's not even in the same league in terms of storytelling and writing. The story in KOTOR 2 is maybe the best Star Wars story ever (I haven't read all the novels). But, I've never seen anything like this in a Star Wars game before.
The dialogue is some of the best ever put in a video game.
The character development and writing where you first meet Atris and discuss your past crimes is better than anything in the first KOTOR.
And the thing was I thought KOTOR was a marvelous game when I first played it, now after playing KOTOR 2, I realized how simple and cliche the first KOTOR was (not that simple is a bad thing, but it's child's play compared to KOTOR 2).