Well, for me, SFMM is the better park as of now (I've been to SFMM countless times, whereas I've only been to SFGAdv once), because I've had some truly amazing days there. My biggest problem with SFGAdv is that it's crowded and operations were absolutely terrible on the day I went. El Toro had a 3 hour wait that should have been one (this was its opening year), and Nitro was a 2 hour wait that should have been one. The park was dirty, and most of the theming had its paint fading and was just plain ugly (see the ice cream building and the cheap, ugly tents).
I know that SFGAdv has improved a lot over this and is probably the better park by now, but SFMM has the better selection of coasters and I've had much better experiences there.