I think that Alan Wake is the more surprising game as it's a new IP with an interesting story, great graphics, maybe a little too simplistic gameplay but it's still good. GOW3 is pretty much the same than GOW1&2 but with improved graphics, you won't be surprised by anything except by the scale and the bossfights.
I'd say that neither of those is a good choice to play with friends though. I'd recommend Alan Wake because it's designed like a TV show, you could discuss the plot with your friends episode by episode, and be wowed by the cliffhangers, but that's pretty much it, it's better to enjoy it alone at night, episode by episode.
You should play both, though, they are among the best games of this year. Personnally, Alan Wake is my favorite mainly beause it's a new IP by one of my favorites developers. I also liked GOW3 though, but there's nothing really new here.
EDIT : oh, OP made his choice, my bad. Well, you should still play Alan Wake later I think, maybe alone would be the better experience after all, I don't really know, I only play single players games... single.