LoL @ the guy thats talking shit about Halo 3 - stfu?
Anyway, as for choosing.. COD4 is more based on real people real war & its cinema and reality will make you feel like participating in something that matters, Halo is nice, but its very futuristic lots of weird creatures sci-fi and spacey shit + Halo is a good game if you played 1&2 if you didnt, than i would advise you to take on COD4 just for the fact COD4 isnt really a continue of COD3 so you start over with a ''fresh'' story. Or you really must be in to sci-fi spacey FPS games, than you should go for Halo 3.
ps. Look at videos of gameplay etc. on IGN.COM after watching a few gameplay movies etc. itll be easier to decide what suits you better.