I've done neither, but I have often read that FFMs are never as good as you'd imagine. You need to find two girls who are genuinely into it, which can be tricky.
I'm interested in trying MMF. Something filthy about a girl fucking 2+ guys. The people who have done it often describe enjoying it (which, as I say, isn't often the case with FFM).
Slightly off topic, there was a guy who was on here who was initially asking about MMF and then posted a pretty good review of him actually doing it. Anybody remember/can link to it? Sorry for being vague.