Between the Skyrim Wiki and UESP?

As far as I'm aware, they're just run by different people (and different people have contributed to its content). It's the same difference as between the Oxford English Dictionary and Webster's. In my experience, the UESP has more reliable and better vetted content and more thorough lore, though.

The UESP is better.

They're both just Wikis about the games, I've always preferred UESP myself though.

I like the layout of UESP better.

UESP has much longer history than TES wiki. It was there before anyone know the word "wiki".

From my past experience the data in UESP is more construction set based and in-game checked, so it's usually more informative and accurate(for most NPCs in Oblivion they even have the daily routine described, like wake up at 6am, eat at 12pm, etc. It's a fun thing reading it).
But those in TES wiki is usually only found in-game by players, so it's not that accurate and informative.
But Skyrim CK has yet to be released so this cannot be decided at the moment.

And of course the lore, there's no other choice except UESP.

I've noticed Skyrim Wikia is better for gameplay, quests, and game specific tidbits of info, while UESP is much better from the standpoint of lore, information about certain things from a story perspective, and spreadsheets of stats on specific weapons/items/abilities/etc in the game.

This is entirely subjective BTW though. I prefer UESP because of how thurough they tend to be in articles, but many times I'd be looking up some information or filler reading that they wouldn't have anything about but the Skyrim Wikia does.

This^
I believe the UESP has way more info as well, especially about the older games.