ferricide:
I think the second game is significantly better than the first (which I also enjoyed).
Aside from improved graphics (thanks to the second game's Naomi roots - the first game was on Taito G-Net hardware, which is essentially PS1), I agree with epmode, the bullet patterns are more interesting than the first. The import version also has limited continues (I think you earn more as you play, similar to Ikaruga) which means that you can't easily credit through the game in an hour.
I haven't played the GameCube version, but I've checked out the DC, PS2, and Xbox versions...not much difference between PS2 and Xbox, but both of those versions are slightly better than the DC version (since you have the ability to turn off the slowdown if you're feeling particularly masochistic - since the DC is the same speed as the Naomi, you can't turn off the arcade slowdown)...