Just built a desktop, have a 7970 and a 670 (got it from a friend), and tested the game with both. I couldn't honestly tell a difference, both ran superb. I know I've seen a few people having issues with the 670, and nVidia in general, but I haven't seen anything that would qualify as an issue with the 670, yet anyway. Running the 306 beta drivers as well (Don't use the 301 WHQL if you plan on playing this game...get the beta's).
I have no reason to doubt you. Both cards perform similar in almost every other game as well, no reason to think this would be different. GW 2 is just demanding at the highest settings, doesn't matter which card you use. My 670 plays the game very well for me. I don't have any complaints since the last beta event when it appeared they did some optimizations.