Super Stardust HD is a lot more like Geometry Wars in that there's a ton of stuff on screen and it will all kill you in one hit (or two if you have a shield). Avoiding enemies and debris is the key, and it's pretty hard.
Dead Nation is slower paced, more focused on shooting enemies than avoiding them, and you get a big health bar and a few checkpoints each level so that if you die, you do not have to start again from the beginning.
I much prefer the style of gameplay in Dead Nation, but if you are good at games like Geometry Wars and enjoy starting again from the beginning when you run out of lives, then you might prefer SSHD. I burned out on SSHD in well under 10 hours because I reached a point where I just couldn't get any farther in the game. I only got Dead Nation today but I already know I'll play it longer and have a lot more fun with it than I did with Super Stardust.