I felt Dead Space 1 was scarier mainly for 2 reasons. It created more suspense, and used alot of scary ambient noises throughout to keep you creeped out. The feeling of being in a small place, and knowing no one is alive, and anything moving is out to kill you etc.
Deadspace 2 was scary in a different way, but you got used to it. In this one, they bombarded you with alot of enemies at once, fast enemies. So youd be scared trying to take all on, but after a while i seemed to get used to this. It had faster gameplay, less like oldschool horror games (whereas Deadspace 1 had all the old school horror survival feel) where an area took quite sometime backwards n forwards. You only ever went forwards in deadspace 2, quickly transitioning into some sort of action based scenerio like the crashing train etc. Scariest moment on Deadspace 2 (i havent completed it, bout 3 chapters off the end... courtesy of the Ps breaking a while back) i felt was the first time you go against those velocorapter type necros. The way the room is deadly silent, and then you see a head pop out. Scary shit. I also liked the level where you heard something running all around you, n they really milked it until something charged at ya.
