CoD and BF both provide a different experience so I don't like to compare the two. CoD is all about keeping a fast pace. The maps are compact, the spawns are close and killstreaks keep the action going. You can hop in and out of games without needing a solid team with you. It's more about just having a quick, fun game with little to no thought behind it.
BF is the exact opposite. The maps are gigantic, the games are long and most require some objective be completed. It encourages teamwork and strategizing with each other. You can't really run around and solo everything like in CoD.
Just different strokes for different folks. I personally like them both but I'd never buy BF4 if I didn't have friends to play with. It's not a fun game to play by yourself, in my opinion.