I prefer m/f clips, almost only buy m/f clips, and from what I've heard from more than one producer, m/f clips sell more than f/f clips despite a vocal minority insisting that "we" all want to see f/f clips. I love a good f/f clip, don't get me wrong, have bought many......but I watch those clips far less.
When the f/f clip features a tickler that knows what she's doing and enjoys what she's doing (jade indica, brooke, kelli lynn sage, MJ, etc etc), it's great. When it's a producer just telling two models to tickle each other (often freeing him up to indulge in crappy camerawork, roaming all over the place rather than staying put and capturing the action), I'm less interested. It's important for me to feel the "tickle lust" coming from the 'ler, the relishing of the tickling, and I find you get that more in m/f clips. Of course there are tons of crappy m/f clips too and plenty of fantastic f/f clips, it's not a hard and fast rule, but in general I prefer the dynamic of m/f vids.
To the OP: the best thing to do is vote with your wallet. There are plenty of f/f clips out there that I'm sure you don't have.