Comes down to which gunset you like. Enjoy modern black rifles with their polymers, high tech sights, and lots of automatic weapons? Go with CoD4.
Like more vintage Bolt action, semi automatic, and slower firing/smaller magazine WW2 weaponry? Go with WaW.
Oh, and WaW has tanks in some MP levels, changes things a lot. Plus a lot of other little changes.
IMO, choose which gun era you like more.