1) One is female... the other is male.

2) Most discussions of rap center around the males

3) Males and females have different physiological makeups, which influences our experience of the world.
3a) (and this is the most important difference for me) Experience of the world influences the subject matter of raps... heavily (such as when I describe how Jean Grae's "Love Song" hit me... a man couldn't have done that or even rap the ending in first-person. It meant more coming from a woman.)
3b)Voices sound different... you know, estrogen/testosterone levels and whatnot

4) Currently, women seem to have to dress scantily clad to get radio spins

Maybe you can think of more