go with COD:WAW... you get the best WWII shooter on the market AND zombies!
I was really anticipating another COD3 when I found out Treyarch was in control again... but I must say (imo) it's the best of the series. The creative presentation of the campaign is brilliant, you finally get involved in the Pacific theater (flamethrower madness), the script is a bit more realistic (cursing aplenty), and a game involving all sorts of automatic weapons and explosives has finally added some much needed gore... Legs and arms go bye bye!
If you're a big fan of Modern Combat, you will be pleased to see the entire ranking and perk system left intact for multiplayer. There are tanks and they are mean as hell. There are also blood-thirsty dogs (instead of choppers after 7 kills) and they are meaner than hell.
Best part of the game though..... ZOMBIES. Yep, once you beat the campaign, you will unlock "Nazi Zombies," a full blown zombie survival mega-minigame that you play play solo or 4 player co-op. I thinks its great. Not a full blown game like L4D, so I'm not sure how long the fun will last, but it's a great addition to an already stellar game.