COD4 is overall better, particularly MP.
Depending upon how you view SP play then you could argue RFOM is better as it has a more traditional 'plot' vs COD4 which has an 'incident' which serves as an excuse for a number of loosely connected (but wonderfully executed) firefights. I know a fair few FPS fans who see RFOM as a better traditional SP experience (even though they prefer COD4 overall).
COD4 does however boast one level, a flashback level no less, that is one of the best SP levels I've played for a long time. It also has the best 'guilty pleasure' level I've played in a while as well, where you rain down a whole lot of death from the skies in a very non-PC (and I suspect unrealistic) fashion.
Try to be reasonable... its easier than you think...
