I'll be completely shocked if Mario 3D Land doesn't outsell Super Mario Galaxy. In fact, I'd say it's more likely to sell more than the two Galaxies combined than not to.
I'll also be quite surprised if it reaches anywhere near the critic reception that the Galaxy games did. In the 5th through 7th generation, no handheld ever made 93% on gamerankings. Ocarina of Time 3D made 94% somehow, so the 3DS already has the highest rated handheld game of all time (and it's the same one as the highest rated console game of all time!)
I think a large part of the reason that handheld games generally score lower is that they don't get the hype-inflated scores that some games (like many would say that GTA IV, for instance, got). I doubt Super Mario 3D Land will change the trend and be superhyped for high scores before release.