Both are great. Very different games though. Both are about the same length, and both have massive replay value. If you play a lot of co-op go with Extraction, if you are a big survival horror fan go with SH. Both titles look awesome as well and are very well acted and produced. The sound and gameplay are also both top notch on both. These are two of my favorite games this year so I would say break down and get both if at all possible.

Definitely Silent Hill! That game looks amazing.

I haven't played Silent Hill yet, but I think I am going to get it eventually (I am a bit hesitant due to the mixed reviews SH is getting a the moment, though).
If you are somewhat interested in the Dead Space universe (sci-fi story etc.) I can definately recommend DS:E. It features great graphics and replay value and a very dense and engaging story. It's a shame it only sold 120k copies so far...

I have played both, Silent Hill is >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Dead Space I mean its not even a competition, me and my friend we played Dead Space and we do enjoy our rail shooters he thought it was mildly entertaining and I thought it was a good game but I wasn't feeling the fun. Silent Hill has amazing everything from graphics to story to new innovative ideas for the horror genre, its a must have.

Disolitude also put some lengthy impressions on the first page of the same thread.