Depends what you're after, you want something with a good single player as well as multiplayer or is multiplayer more important to you. Ace Combat is good offline but not much fun online, there isn't many people online in that game, Burnout is fun, that's a given, BF:BC is not as good imo as BF:Modern Combat which was online perfection in fun and playability.
Other games I would recommend, hmm I would probably say pickup Rainbow Six Vegas 2, brilliant story campaign that you can play in coop and the online is great fun. Or why not take that £20 and trade in some game towards Fallout 3 or Fable II or something.
In the end it comes down to what you like, I won't be buying GoW2 because it's not my kind of game but I know others will and that's fine, just like I like some games and others don't, it's all down to if you will enjoy it, that imo is the main thing.