So I decided to just buy Geometry Wars on Live, and... I sort of regret it and I sort of don't.
On the one hand, it's cool to compare to GridWars. The sound is vastly superior, and the graphics are superior in certain circumstances. It's just as addictive and I'm totally up too late again from playing it. I said, "Okay, one more," and "No, that one didn't count," and a billion other things multiple times. And, of course, it works when you want it to and you don't have to screw with it.
On the other hand, GridWars is practically just as good. They're nearly identical at face value, but when you look at it a bit deeper, GridWars may well be the superior of the two for some. It isn't quite as ridiculously hard, which may be good or bad depending on how you look at it, and the pacing is different. Geometry Wars seems kind of... scripted. Stuff happens in an order and you have to go through the order. It seems randomish and all because of the chaotic gameplay, but you always know that all the blue diamonds are gonna' spawn from the corners after you play a bit, or the green boxes will do it, or the crazy snake things are going to come out in droves. I didn't get that feeling at all from GridWars which seems much more random. Also, GridWars felt like it was paced a little better. There's a few seconds more downtime after you die that I don't like so much, but then there never feels like there's a ton of empty space while you wait for things to spawn. With Geometry Wars, you have to fight through the really boring early stages every time you start a new game and that kind of sucks after the chaos of the later game.
So I have mixed feelings. I also feel dirty for giving my credit information to Microsoft. The marketplace stuff *is* slick, but it's... weird. I don't know. I just can't quite find myself digging it. I like my 360 better when the cable plug is in my computer. Being "connected" just makes me feel like a tool. I don't know.
Anyway, if you're having doubts about whether or not you're missing anything with GridWars, I'd say you should put those fears to rest. It's apparently a pain in the ass sometimes, but if you can get it set up with a nice dual analog controller and stuff, I don't think you're missing much.
Still... those black hole things just look amazing in Geometry Wars. Pretty satisfying in GridWars too, but there's just a little something extra there, heh.