I have mostly mares. At this point in time I own 3 mares, a 2 month old colt, and a gelding. My mares are very good about not showing much in their heat cycles, even in a herd together.
Now. I have seen some RANK mares during their cycles. Oh my gosh. A mare that has a bad heat is something to stay away from. But in general, I don't understand the aversion some people have to mares.
Plus, if you want to go beyond the temperament part of the equation, a mare is worth more financially than a gelding. If a gelding is injured and can no longer be ridden, he is basically done as anything but a pasture puff. A mare can still be bred if desired, so a lot of people prefer, even refuse to buy a gelding!
In my opinion, if the horse fits (aside from stallions), then it doesn't matter to me if it is a mare or gelding!
Riding: The art of keeping a horse between you and the ground.