I've played both franchises, and to be honest... COD sucks!

COD (Black Ops)

Cons

-It has a severely unrealistic engine, a very mentally challenged fanbase, easy camping, EXTREMELY expensive add-ons.
very cheap color palette, the idiotic "Perk system", countless anachronisms, VERY small multiplayer maps, and last... VERY boring online experience.

Pros
Great story.

Battlefield (Bad Company 2)

Cons

-Occasionally glitchy/laggy online play, Story could have been better.

Pros

-Hilarious characters, natural dialogue, hiring actual Russians to do the Russian dialogue.
Realistic gameplay, room for creativity with destruction, impervious to campers, realistically distinct gun sounds. Unlike the "perk system", BF's "Speacialties" are realistic, such as "Alt Weapon" which enables you to use other weaponry in vehicles, the various atmospheres in the game have different coloring (Pale palette in deserts, fresh and bright in jungles... etc.) which in turn supports the "realism" theme.
A great selection of vehicles, MASSIVE online maps, encourages the use of strategy rather than camping/spawn killing, encourages teamwork with the "Squad System", rich engine. even in the largest of the maps (Atacama Desert) the gameplay is still fast paced and balanced, very fun to play online.

All the moronic COD fanboys are saying "COD id better because it sells more"

Toyota sells more than Ferrari, does it make it any better?

QUALITY (BF) VS. QUANTITY (COD)

Heavy Metal, My Lifestyle.

Nuff Said!