I think most people are forgetting why games sell, most the time it isn't due to one being 'superior' over the other.
As suggested, Halo has always had a huge hype around it. Personally I own Halo 3 and I'm not half bad at it, played Halo 2 and haven't touched the 1st installment. I agree with Tsukasa mostly, the single player campaign was near to pathetic but it made for great fun again with multiplater.
The multiplayer modes on Halo were and always will be the main selling point of the game, which is why it probably sold so much. With XBL and all the rest of it I think people are swaing to competitive games online such as Halo and CoD4 rather than the likes of the good old games such as Devil May Cry 4 and dare I say it? *In fear of getting screamed at* Alone In The Dark 5.
To be honest, I pretty much love Halo but I've seen what is coming with GoW2. To answer your original statement I don't think GoW2 will stand a chance sales wise against Halo, but it will still be pretty close.

(Watch me get proved wrong come November was it?) I'm sure we've all seen the Tech demo and it seems the entire GoW2 environment is wholly unchanged despite a few additions to guns or new 'abilities' such as holding a Locust as a shield. The game engine is pretty much the same I think but they've changed material properties.
Unfortunatly, I am probably being biased towards Halo as I didn't get the original GoW for ages and the online mode is pretty much dead...that coupled with GoW actually sucks because I got to Act 4 on Insane then it randomly placed me at the beginning of Act 1 on hardcore without even the option to play Insane anymore...
Still, if GoW2 can build up the hype maybe it stands a chance, but think it like this Halo is a cult game. You wouldn't see Batman merchandise get compared to stereo sales...But you'd definatly see Batman merchandise get a fair competition in relation to FF sales.