Not ALL the stages are linear. Most of the ones with multiple planetoids are, but a few galaxies are large areas not unlike areas in Super Mario 64 (the Honeyhive Galaxy is kind of like the big floating island that many SM64 levels were with normal gravity, for example) with a few little planetoids around them accessed via cannon.
One big plus SMG has is three Bowser battles. Remember in Super Mario 64 when you had Bowser in the Dark World, then in the Fire Sea, and finally in the Sky? Super Mario Galaxy has a similar setup--you'll fight him three times, defeating him in a similar manner every time.
It's close between 64 and Galaxy. I think I'll have to go with Galaxy for the many many galaxies. Looking back, playing the same level six (eight or even TEN in Sunshine!!) times got pretty boring. Galaxy has you play the same level three or four times or less, often not going the same path, and then you move on. Later, you'll come back to those galaxies to play for a star triggered by the "Prankster Comet" or bumbling Luigi.
And. Powerups. The one mentioned but not named in TMK's review: the best.