For fiction, one usually aims at a broad market. The trick, I've been told, in written fiction at least is to try and avoid describing things that might cause either group to drop their willing suspension of disbelief. It can be harder in visual arts since things you can fail to describe in written work, e.g. whether helmets have bull's horns, are a decision that has to be made for the visual designs. Of course the cover artist can then screw you - see Charlie Stross' "The Family Trade" for an example of that.