BE is the bomb. (lol) I have a 51 BE warlock and it's been a much easier transition with the help of the mana tap and arcane torrent which BE have as a racial passive. Very handy. ;) As for UD...I've also got a 24 UD warlock and between the two I'd go with BE. Makes getting back your mana much easier with that mana tap and arcane torrent. BUT, be warned -- the mana tap will sometimes aggro things...and when you use the arcane torrent make SURE you don't have several monsters nearby...otherwise you're going to get attacked by 'em all.
My suggestion in choosing a BE warlock...learn your class well and make sure you know your race well too. No matter which race you choose it's always best to learn all you can about it before you get into any real battles (as in BG or PvP).
Most of all, have fun with it. Learn your race, your class and your minions and their abilities. Once you have all that down, you're unstoppable. :D
-- Challis (Tortheldrin - lvl 51)