I liked Super Mario Galaxy 2 much better than 1. Maybe because I compared 2 to 1 and not to 64, as I did with the first Galaxy. Maybe because difficulty was a bit harder on the second game, I don't really know.
The only category in which Galaxy surpasses Galaxy 2, imo, is music. Super Mario Galaxy's soundtrack was awesome and very memorable. Galaxy's 2 soundtrack didn't fulfill my expectatives, actually, I think they kind of spoiled an awesome job previously done.
But then, level design in Galaxy falls short. That's something that was improved in Galaxy 2, imo.
I might be the only one here, but I do miss the punching and kicking ability of SM64. Throwing stars is lame, if you are not a ninja that is.