Not to start a debate on which WoW Emulator is better, but I have been following the Ascent (previously Antrix) emulator scene. Though I was majorly disappointed at the dramafest they recently went through with WoWScape and something about donations for favors (kinda sounds like dating to me...), I still think that emulator has the most potential for scalability. I've run both, and I just like the way Ascent is put together from the inside out.
Anyone here is welcome to try it out if you like. Follow the link in my profile to create an account blah blah. If I like you, I might even give you elevated rights
(since there is no support whatsoever, it's just a dev/test server). I usually update it when time permits.