I was just testing both recently and came to similar conclusions. Airfoil is great in theory and the flexibility for many differnet programs is a valuable feature. Whereas RSO is less flexible, dedicated to work with a few music players only.
BUT in use I had two major issues with Airfoil;
1) As I change songs through my list, I start get a buffer problem of some kind and the songs don't start clean. They skip, burp, start late into the song. No good at all.
2) To control volume, I have to click away from MM and go to a volume control window, either for my system or for Airfoil, as it has a helpful option to use system control. A minor issue, but annoying. I want MM volume control to work for quick adjustments within the window and with Airfoil it doesn't.
Rogue Ameoba is very helpful and tried to work with me on the first issue, but I quickly ran out of time/patience when I saw that RSO works.
RSO, in use, has been seamless. It's the one that gives MM the AEX output like iTunes. Volume control works, pause, play, changing songs works. RSO is the one I paid for.
I have found a minor problem in use, which is high CPU usage while in pause (not in play!). But it's not a deal breaker and I suspect it's a plug-in, script or version conflict or something like that. It seems like it'll get worked out.
So, RSO won for me. I'll try airfoil again down the line and maybe get it anyway for playing videos.