I've only played Geometry Wars: Galaxies (for DS and Wii, the games are 99.9% the same on both versions). There are different levels (board shapes, variance on enemies, medals awarded for different scores) and an upgradeable drone. The geoms you get from shooting are used to purchase new levels, and abilities for your drone.
It's a nice game to sit and phase out for a bit when you have time to kill. I don't think I wouldn't be playing it as much if it wasn't for the variety in levels and gaining the drone experience points.