I have never done an all-girl threesome, but it certainly sounds like sensuous fun. Most of mine have been MFM since I began occasionally doing these in my teens -- having two guys in absolutely wonderful -- they certainly keep me busy. As long as they keep me at the center of their attention, I could keep a good MFM going for many hours.....
I had a few FMF threesomes in college (with my roommate and several of the guys we both knew) and they were wonderful. I really got to like heaving my face and head down by the action between the guy and my roomie - I loved watching his cock plunge in and out ... and then cleaning her out and up when they finished.
But if I had to choose, I'd go for a nice MFM.