If you want the Geometry Wars that feels the closest to Retro Evolved on the 360 in terms of difficulty, the DS version ain't it. On any of the BAT planets, and in some of the later stages the slowdown begins almost immediately. Nowhere close to as much spawns on the DS version either, on more than just one planet have I been able to survive for more than 2 hours without any effort whatsoever and place in the top 5 on the leaderboards for those stages. Speaking of which, the DS leaderboards on a whole have FAR higher scores than on the Wii because of that difficulty difference. On the DS the green and pink squares are no threat at all, on Wii you will FEAR them because on there just about everything moves faster, comes at you in much higher numbers, and you get no slowdown assistance in dealing with it at all.
Other than that, for kicks on the go the DS version is quite nice. Just don't expect to feel the same level of dread that you do on the home game.