I honestly preferred Super Mario Galaxy over Super Mario Galaxy 2. Super Mario Galaxy just seemed way more original for me, and Super Mario Galaxy 2 felt almost as if it was the exact same game with different levels. I mean, the story is virtually the same as it's predecessor (Bowser tries to take over the Universe), the music tries to sound similar to the first game, only in a different key, so on and so forth. The only significant improvement was gameplay mechanics.
The addition of Yoshi was quite nice, but for some reason, they took out the whole sort of cosmic feel of the game. While the gravity still exists, Super Mario Galaxy 2 isn't as, um, planet based as it's predecessor. Most of the levels are sort of flat or walls, not spherical planets and galaxies. I'm also perhaps one of the only people who doesn't like the whole Starship Mario and the Super Mario Bros. level select. Rosellia's ship definitely felt as if it could be a part of the plot (in fact, it was), whereas I have no clue why they would just turn a satellite into Mario's head, which I find quite difficult to walk around.
I might sound like a person who doesn't like sequels that much (don't get me wrong, I like sequels if done right), but the original is definitely better than the first one.