Dead Space all the way.
Silent Hill is okay (and you'll probably find yourself stuck somewhere or frustrated and almost to turn it off as I did numerous times). I'm honestly going to trade it in toward Fallout 3 next week.
Dead Space on the other hand is the best survival horror game I've played since Resident Evil 4 (screw the naysayers - RE4 was a great game).
It's dark and creepy and bloody and is great for long late-night game sessions in the dark with the sound turned up.
Dead Space should outsell Silent Hill when it's said and done.