Only reason I'm asking is I want to buy one of those two from games on demand since their both only $20, but I can't decide which.  I owned Dead Space for a while but didn't really get into it, I probably played about a quarter of the way through it and just kinda stopped.  I've never played that Prince of Persia so it would be completely new for me but I'm not sure based on what was said about it when it was released.  Length of the game is a factor since I'm really bored lately and whichever one could entertain me longer the better, but that's not completely what I want to base the purchase off of (since I think Dead Space is a good bit longer than PoP).  So I decided to put the question out there.  If you could only get one, which would you choose?

Dead space is a game I loved. Play it on a harder setting for real fun. 

If you didn't really get into dead space the first time, it might not be that different. Try playing PoP for a bit

Dead Space is an awesome game so I would go with that because I am not sure which Prince of Persia you are talking about.

PoP is easy, easy, easy, peasy. 
 
It's fun, for sure, and beautiful. I really liked the voice acting and most of the story, but gameplay-wise it's not much challenge. (If that's important to you)

I bought PoP and would unbuy it if I could.  it's not bad...  I haven't looked at it since.

Prince of Persia.

Anyone who says Prince of Persia is out of their damn mind. I'm looking at you, buzz_killington.

I really liked PoP; somehow the game felt longer than it was, in a good way. I haven't played Dead Space, though, so I guess I can't judge.

Dead Space, although it is a bit short, but it's great.

I really like both. But if I can only have one, Dead Space.

" @c1337us:  PoP 08 I guess I can call it.  The one released with the awesome new art style that was apparently super easy. "

Both, man! I loved both, but I'd normally give the edge to Dead Space, HOWEVER, if you didn't like the first quarter of it, you're not going to suddenly change your mind and like the rest of it. Go with Prince of Persia. It's great. Just be aware that it's gameplay, while not bad, is simplistic and likes to hold your hand. Still, it's a fun, beautiful ride while it lasts.

I loved Dead Space, but like you I was kinda iffy on it about halfway through. Sometime after that I really got into RE. So, when I went back to Dead Space I had a whole new appreciation for it. Bottom line: if you like RE and that sort of game (Alan Wake as well), then go with Dead Space. All in all, it's absolutely fantastic. 
 
As for PoP, I'm actually just playing it now. Seems more of a casual experience - lots of running, jumping and exploration. Far cry from the horrifying yet linear Dead Space.