I've had them for three hours now...and I've been switching back and forth between the two.
In a nutshell, everything about the F1 is just a little better, starting with with build quality and materials used. The F1 feels more solid, and it gives me the the impression that it will last longer. The cable is a little thicker and the plug is much heftier.
The F1 has less mid-bass and more sub-bass. The highs are a little rolled off on the MA900, compared to the F1, and I can sometimes hear a little sibilance with the newer Sony. The sound stage is much wider on the F1 (much wider) -- music sounds more open and realistic. It really gives you the impression that you are listening to speakers. Finally, the F1 weighs less and it is more comfortable.
The MA900 is good, but the F1 is great.