Good question. I am also looking at both of them and cannot decide which has the correct information sometimes. UESP seems a bit better though in my opinion, although I find it harder to navigate at times.
One thing that has been drawing me to UESP is the effectiveness of its search tool. It seems more to the point, taking me directly to desired information. Wikia just seems more fluffy, better looking but not as in-depth. However, one thing that Wikia has that I've been finding extremely useful: because we're in the early stages of Skyrim, there are lots of useful workaround tips for bugs in the Bugs section. It saved me a lot of pain, especially questwise.