The battlefield series is in the name-its like a real battlefield. Its epic in everything it does - the scale of the maps,the weapons,the destruction, the teamplay etc etc.
COD - a bloody brilliant shooter, but its more for the 'arena' type player, who just wants to run around and shoot.
Seeing that you dont have ADSL, I would say BF3 is out of the question. Battlefield is traditionally an online multi-player FPS, the campaign is simply an element which came further down the line.
For COD, the fun is also mainly online. Online play is the reason 80% of people buy these games. However, in my opinion and from what I've read, COD provides the better campaign (which is all you will be able to play since you are not going to play online).
Just a heads up though, since these games are essentially created for online play, the campaigns are only about 5-7 hours long, after which your R500 is depleted.
My suggestion - go for neither. Rather get a game which is entirely based around a campaign, so you can get your money's worth from hours and hours of play (for example dragon age which is all about the campaign which will last you up to 30 hours). If you do however see yourself getting ADSL in the future, get BF3.