Well, I've started mares, geldings, and stallions and honestly I'd have to say that it depends on the horse, and it's innage personality and intelligence. My stallion is soooo layed back and well disciplined that most don't even realize he's a stallion. (until they look! 8) ) He tries so hard for me and usually picks up on things very quickly.
I've had mares that are witchy, but they were always like that, and others who were so easy to train it amazed me.
I've also had stubborn geldings and geldings who were not the sharpest tools in the shed and was really glad that they were in fact gelded. (don't want to pass that on!)
So, to me, it all depends on the individual horse. Not some stero-type.