I think adding genders is a great idea, and I wish more games allowed for it, or multiple playable characters in general.
I've found as a rule that the more character customizability you have, the more unique branches of dialog and things you'd have to have to accommodate the variations. So, you need to have the differences between the characters be very minor, have a very basic story (making the character differences only visual), or do a lot of work to provide a unique experience with each character. I opted for the third approach when I rewrote all the story scenes for Hitodama twice over. 
Anyway, I know you're going for mostly a visual difference and am not expecting any big differences in dialog or anything. Jetbox's question just got me thinking about this stuff.