Super Mario Galaxy 1 and 2 are a typical example of how Nintendo works, really. They do something that's new and different with one game, and then they stick to that formula, gradually making each game better. The sequels lose the lack of awe that the first game gives, but the small improvements everywhere offset it. Then, after a few sequels, they radically change the formula again.
Other examples of this are the Fire Emblem series (well, post FE6, at least), the Zelda series (at least with OoT to MM to TP) and the NSMB series.
To me, the games give different kinds of enjoyment. Super Mario Galaxy 1 gave a constant feeling of awe and excitement, while Super Mario Galaxy 2 was much more laidback and feelgood.
This is, again, similar to the difference between Twilight Princess - which was more like Super Mario Galaxy 2 - and Skyward Sword - which was more ala Super Mario Galaxy 1. Or, in the Fire Emblem series, Radiant Dawn threading new ground and being like SMG 1, while the two DS games are more ala Super Mario Galaxy 2.
They fullfill different roles. I usually prefer the "new and exciting" games, and that's the case with the Galaxy games. I put SMG 1 slightly higher than SMG 2.
