Both games have very distinct aspects that lend a very unique and personal touch to both of the games. It's a hard choice to make but I'm sure that more people will be able to appreciate both games more than your average FPS game.
CoD4 is a great single-player experience as well as a more than enjoyable multiplayer one. The online multiplayer is a amazing feat and offers a lot of re-playability, customization, and fun. The game has a rather relaxed ranking system which really pulls through as you play through the ranking system time after time again on the prestige mode.
The ranks are spread out enough that they won't be achieved every other game, but not so much that you can't see any noticeable increase from the ranking. The ranks add a lot of depth to the game and the hoppers inside of the game allow you to quickly find a match and have a great playtime to wait time ratio.
Although the online is good the local play leaves a lot to be desired, with no co-op or online split-screen the game doesn't lend much to having people over. The achievements were 100% single-player though and were evenly spread throughout it. A good list overall, not too difficult but not overly easy.
CoD4 is definitely the more fast paced of the two games and is focused more on a continual offensive push against each other rather than a defensive stand still.
RBSV2 has an enjoyable story as well as deep customization options all balanced out with a fine multiplayer. The local play allows for some great co-op games as well as terrorist hunt which will allow you and your friends to enjoy the game. The customization is a great improvement over any other game on the market at this time, changing your avatar to reflect exactly how you want it to look is a great reminder of the fun you can have from editing appearance.
The ability to take cover on walls works very well in the single player campaign but when it comes to multiplayer it makes defending far too easy to accomplish provided that you are camping on a narrow hallway. Unless the player has prior knowledge of your location and a frag then without a highly coordinated strike no one can make a decent offensive strike.
The achievements are rather difficult when you consider the longevity of the game and then weigh it against the achievement. With about a 600 to 400 point favor of online most people are going to need a gold membership to score higher than a 50% on this game. Unfortunately unlike CoD4's ranking system the ranking system feels more like a chore than a add on. The ranking system is defiantly deeper but with more total EXP required and fewer levels trying to get from one rank to the next feels like a task for the hardcore gamers.
Overall I would pick CoD4 for it's online play. If you're more interested in a single player or local play experience than RBSV2 is definitely more suited to your task though.