If you can only choose 1 out of the 2, I'd say Battlefield 4. The game has more depth and more options of play style, so you wont get bored of it as quickly. Full-map 64 player vehicle-dominated conquest large is entirely different from 24 player domination on a close quarters infantry-focused map. Good variety.
However, if you can easily afford both, than I would certainly recommend both. I'll be getting both. I currently play both BLOPS2 and and BF3, pretty much equally. BF and COD are really not as similar as people make them out to be. Yeah they both generically fall under the FPS genre, but they are still totally different. It depends on mood. Sometimes I'm in the mood for the frantic, twitch-based, very fast paced infantry-focused combat on tight, claustrophobic maps. COD will give you adrenaline rushes, and the ability to blow away a ridiculous amount of enemies in a short amount of time. At the same time, it will probably also frustrate you greatly sometimes
When I'm in the mood for more mellow-paced, somewhat tactical gameplay that depends more on skill, thinking, and teamwork instead of luck, Battlefield gives me what I want. When I do well in Battlefield, it gives me a greater sense of accomplishment than COD does, since there's more of a learning curve.
I know that COD is usually portrayed as the scum of the gaming world and Battlefield is the "savior" of FPS games but both of those views are very exaggerated. Yeah, the Battlefield community tends to be just a little more mature on average, but there's definitely times the BF community sinks just as low as the COD community. And COD Ghosts is shaping up to be a bit different from any other COD game ever made. Still the same game at the core, but a lot has been done with Ghosts to make it play like a completely new branch of the COD series They're also putting more effort into the PC version than any previous COD game as well. For the first time, they've brought in a graphics chip maker (Nvidia in this case) to help make the PC version be the best looking out of all the platforms as well as optimizing performance. IW's executive producer says they'll release rentable ranked servers and a server browser shortly after launch and they're working on release mod tools as well.