I'm actually completely torn between these two series. They've both managed to give me a chill, and have really touched on the Sci-fi and psychological aspect of things.
The RE series has always entertained me, as i'm one of these "Zombie Enthusiasts". You know, the one with the plan of action in case of a zombie apocalypse. You know...just in case. o-o The two way battle between the protagonists and Umbrella Corp + the zombies/mutated beings is always entertaining to experience.
On the other side of things is Silent Hill. The dark, creepy, twisted horror series that plays more on the psychological aspect of things opposed to the straight line of combat in RE. Though, combat is also present in Silent Hill, and is in my opinion more entertaining and satisfying. The story of Silent Hill is also a very interesting detail. I love the little backstories of the various deities, orders, locations and creatures that feature throughout the series. There's always a twist or reference hidden somewhere throughout, and the various enings never fail to suprise/amuse me. That is, amuse in a dark way.
Basically, there are aspects of both that appeal to me, and it's hard to put into words my love for these two series.