It comes down to your style, i guess WaW is "techinically" better, but i prefer CoD4.
You can't really ask a public forum which is better, it comes down to YOUR prefrence.
Personally i like CoD4 because of the competitiveness and it does seem like there aren't as much 10-12 year olds screaming on the mic, and obviously becasue of the lack of the Mp40 helps. But mainly i like CoD4 more because of the atmosphere?

IDK i just like the feel of CoD4 more.
But WaW is good, to many people bash it because of the Mp40, which is true, but if you skill level is high, you can easily take them out like DG said with a pistol, I've seen DG get double digit kills in SnD with only using his knife.