--Quickly typed this up, excuse errors. Didn't proof-read.--
It wasn't much of a battle before this year, but with Modern Warfare 3 and Battlefield 3 being released less than a month apart from each other this Fall, it's really started to heat up. But which franchise is better?
If you ask me, it's a short argument. As with most disputes between two things, it's a matter of personal preference. For that reason, it's hard to really say which is better - especially since the two games really do differ quite a bit.
Similarities first, they're obviously both modern war-time FPSs - that's essentially where the similarities end. If you've played both of these games, they play completely differently. Battlefield makes teamwork a necessity, while in Call of Duty, you can hop in a match of Team Deathmatch by yourself, play without a headset, and do quite well. If you aren't in a good, communicative squad on Battlefield, you can bet your @@@ is going to be had more often than not.
It's not to say Call of Duty doesn't require tactical skill, because it does, but in comparison to Battlefield, it's not even close. Call of Duty's maps are minuscule when sided against Battlefield's, and in most game modes, allow for 12 players tops, while in Battlefield 3, you can have 12 per team. While Call of Duty has more gametypes that require objectives to be completed online, it's more of a challenge, and feels much more rewarding in Battlefield for the most part, for that reason.
It may be stupid of me to say, considering I prefer Call of Duty to Battlefield, but it's just the truth. Battlefield is a much more cerebral game overall than Call of Duty. As you've probably read elsewhere, Call of Duty caters more to the casual player, while hardcore players will tend to maybe prefer Battlefield. It's not to say there's only one way to go - there are plenty of people that play and enjoy both games.
In short, again, it's about personal preference. These two franchises are really quite different, and there just aren't that many similarities between the two. Tend to play alone? Go for Call of Duty. Like communicating and teamwork? Battlefield. Either way you go, you'll more than likely have a good time.