I have played many Call of Duty games and they are a good starter game for people who have never played first person shooter games before but after a while it gets pretty boring because the multi-player isn't that fun. It has very small maps and the only people who seem to play are the pros and the little annoying kids. The multiple different guns and customizing abilities were pretty cool too but the gold and diamond guns were pretty dumb looking. Call of Duty is a realistic game in an unrealistic environment.
I have also played a lot of Battlefield and the game starts out much harder than Call of Duty because there and so many different things you have to get use to before you can start winning anything and if you don't start with everyone else you will get left behind and it is hard to get good at this game. It focuses on the terrain that you are fighting on more than most things because there are multiple different vehicles that have to be able to maneuver around the map. Battlefield get more updates and has way more DLC.
Here is a video than goes even farther in depth than I can explain.
Schulz, Chris. "Game Reviews: Call of Duty: Ghosts v Battlefield 4 - Entertainment - NZ Herald News." The New Zealand Herald. N.p., 14 Nov. 2013. Web. 05 May 2014. <http://www.nzherald.co.nz/entertainment/news/article.cfm?c_id=1501119&objectid=11156474>.
Senior, Tom. Call of Duty. 2012. PC Gamer RSS. Web. 05 May 2014. <http://www.pcgamer.com/2012/08/15/call-of-duty-black-ops-2-screenshots-show-tight-multiplayer-scraps-on-futuristic-maps/>.
Battlefield 4. 2013. Viewing Gallery For - Battlefield 4 Gameplay Screenshots. Web. 05 May 2014. <http://galleryhip.com/battlefield-4-gameplay-screenshots.html>.
Battlefield 4 VS Call of Duty Ghosts - Inside Gaming Daily. Perf. Inside Gaming Daily. YouTube. Machinima, 10 Oct. 2013. Web. 05 May 2014. <https://www.youtube.com/watch?v=kj1XErafdeM>.