I wrote an article about this topic around 2 years ago (I'll include the link below.) I think most people should have both sexes for instructors if they are partner dancing. There are a few instructors who do know both sides very well, but few do both sides outside of teaching. Meaning if they go social dancing, they primarily spend the time in one role or the other. And if you find someone who does both well, then after 6-18 months, find another quality instructor to provide you different points of view. I'm not a fan of only having only one instructor must past a year or two, as each instructor brings different points of view and experiences to the lessons. Here's the article if you want my extended POV: http://www.unlikelysalsero.com/2007/07/same-sex-or-different-for-privates.html
I've found that you need variety when learning. I started learning from a female instructor. While she was a good teacher and I learned a lot from her, it wasn't until I started taking classes lead by a male instructor, that I really improved my styling and lead techniques. I think it is generally important to be taught by someone of the same sex, as they share the same perspective on the dance as you. But equally its important to see things from the other side. As a lead, if I understand how the follower expects to be lead, it can only make me a better dancer. I'd also advocate what others have already said here, you should try to attend classes taught by different teachers. This is for the reason why you should mix up partners when in a class. If you don't have variety you can start to stagnate, pick up bad habits, and eventually find yourself repeating routines. The classes I use to attend had regular guest teachers that would do wonders to spice up my dancing repertoire.
I have only had a male instructor. Once, during a group class, two females tried to lead me, and they failed miserably...so I guess it all depends on the skill level.
