I don't know about CoD3 because it was a while ago but I liked WaW more than CoD4. The singleplayer gameplay had more original scenarios, the co-op was a nice option, and even the story was more interesting. Especially when you were the Russian.
The multi in WaW is hands-down better than 4 though imo. The lower "stats" of the guns force more teamwork since nobody can be a one man army as easily and the perks feel like they make a more tangible difference imo. I can understand if some people didn't like the setting or the lack of a strong "all-in-one" gun for every map but I think Treyarch was on the right path with the changes they did in WaW.
The multi in WaW is hands-down better than 4 though imo. The lower "stats" of the guns force more teamwork since nobody can be a one man army as easily and the perks feel like they make a more tangible difference imo. I can understand if some people didn't like the setting or the lack of a strong "all-in-one" gun for every map but I think Treyarch was on the right path with the changes they did in WaW.