HighTreason

While differences in sexual organs are unlikely to affect your campaign much, and I doubt your PC's will be getting pregnant, there are actually physical differences you might want to keep in mind as far as characteristics go. Women are, of course much weaker than men. While this is common knowledge, it might surprise you just how much weaker women are than men. Women would have a slightly higher dexterity, a slightly lower speed, and maybe some other things I haven't even thought of. This all depends on how realistic you want to get, of course. It has already been pointed out that NPC's and perhaps even PC's will often have a different reaction to a male or a female. If your campaign is role play heavy, this could be a very big deal. That is assuming of course that the character actually looks discernibly feminine in female form and discernibly masculine in male form. If they actually look the same but just some sexual organs change, which are presumably covered by clothing anyway, this becomes a non-issue. If it were me, I would feel compelled to go with Multi-form on this in most cases, the exception being a case like the character is not human and members of his/her race do not have any differences in strength and characteristics when they switch genders and do not look any different outside of sexual organs. In that case, I might just go with a Multi-power if the change is instantaneous and can't be stopped somehow. It's just a special effect at that point.