Hi Janeway and welcome to Pet Talk!!
I've always heard that it's good to have mixed sex multi-cat households. So maybe a little female is in order? Actually, I really just think it depends on your 2 cats' personalities and how you think they would react. I would guess that either a male or female kitten would be just fine, especially since older cats are more tolerant of kittens. LOL I'm sure the you'll find the right kitten at the right time, and everything will work out great for all of you...male or female! Good luck and let us know about your new addition!