WoWhead is so much better. It makes great use of AJAX, no ad's, and all-around better display of data.
As nice as AJAX is, it's dirty and should be avoided when it isn't necessary (or highly beneficial). The javascript objects it uses were never meant to be used in the way that they are during AJAX routines. Now, I'm not saying wowhead overuses it (primarily because I haven't visited it in a long time), but you should know that before you come to big an advocate for that reason.
I like alakazam or w/e it's called.
I found wowhead to be a lot cleaner. Thottbot is slapped together and alakazam seems like it was pieced together from fragments of similar projects.