Battlefield may be strategy but it's slow paced. COD is all about your reaction time and is fast paced. Personally I like each one for different reasons. I love fast paced COD because I do better with switching targets fast and using what ever is at my disposal. I like battlefield for it's realism and the recoil (although the recoil pisses me off sometimes lol).
People often choose a side, but why? They are essentially rivals but each game is different in it's own right. So it's hard to compare them properly.