Warcraft 4 by far. There's virtually no reason to make a WoW 2, because they can just keep adding on to the current WoW with expansions like they have been.
Warcraft 4, on the other hand, would be amazing. I don't really care at all about the campaign or story, but custom games on WC3 are absolutely amazing, and I'm sure that WC4 custom games would make it far more fun than any WoW 2.
Besides, if they actually released a WoW 2 and I had to make new characters and stuff there's no way I'd even waste my time with it, would just play another game instead.