I personally enjoy both, but the thing I have found is roleplay is my main focus. For example, I have RP'd my charaters for a few months now, and I have written 4 small stories.
Ofcourse, with the RP I have done, it is easily to write more and expand on the RP, ei, more actions to me mentioned, thoughts, the list goes on. But RP doesn't prove useless, remember most of RP isn't scripted, so it's always nice when your taken surprised by another charaters action, which keep it ineresting, writing a long story still in development, ( for me personally) is pretty boring mainly because you know what happens, and know everything, RP doesn't allow you to know EVERYTHING.
So yeah, RP trumps Fan fiction.