Personally Call of Duty was my first love. The first two games in the series had more personality and were more fun than the wooden Battlefield 1942 and its expansions. However, starting with Battlefield 2, the scales starting tipping in favor of Battlefield. With the modern-gen games, I can't fathom why anyone would prefer Black Ops or Modern Warfare 2 over Bad Company 2, considering the latter has completely destructible environments, a large selection of vehicles, huge maps, and Rush is probably the best multiplayer mode ever created to date. Once I tried BFBC2 multiplayer I never looked back. All signs point to Battlefield 3 being absolutely amazing, while Call of Duty hasn't been interesting to me since the first Modern Warfare.
Ultimately, Battlefield wins out for me.
Ultimately, Battlefield wins out for me.