This game will blow your mind. It will be amazing to see that such a big world is in such a small console.
Super Mario 3D Land is the closest you will get to an open-world portable Mario game. You can't compare it to Galaxy or Sunshine, because it still goes by the World X-X system. However, you can go back and play the worlds any time, just like New Super Mario Bros. or New Super Mario Bros. 2.
The game, unlike the other DS series Mario games, is omnidirectional and 3-dimensional, and I'm not talking about the 3DS's fancy stereoscopic 3D, I mean that it plays like Super Mario 64.
I don't truly think graphics make the game, but it doesn't even matter here, because this game's graphics are about the same as Super Mario Sunshine, however toned down for the small screen of the 3DS a tiny bit.
I recommend playing this on the 3DS XL over the 3DS to fully enjoy the environment, but it's still great on both.