As someone who has done both (FFM and MFM), I can say that MFM is much better.
If you have one hot (and freaky) girl and two guys who are both really into her, you are guaranteed a hot time. All of you who are afraid of "crossing" swords should get over it. If you find the right people for it, you will thank me.
Also, you shouldn't bring ego into it at all. Just try to enjoy yourself and please the girl. You will end up doing both. Size also doesn't play as much of a role as you think. Usually, I was always the larger partner, but the ladies seem to like the contrast... So if you are much smaller (or bigger) than the other guy, I wouldn't worry.