If I had two horses standing side by side, identical and ideal in price, temperament, conformational suitability, and history, one a mare and one a gelding, I think I would pick the gelding. I've ridden a lot of mares, and they all just seem a bit quirkier than the geldings. Into the bargain, you never have to worry about an accidental breeding or other catastrophy. Geldings, for the most part, are just more uncomplicated in my experience. One of my good (guy) friends owns all mares (not through any voiced preference, just the way things worked out) and gets along extremely well with them, but I have definitely witnessed occasional "mare" days. My geldings have off days as well. They're like people that way, I guess. For sure visit all purchase prospects in person, twice if possible to cut the chances if just seeing a "good" or "bad" day. I like horse shopping in the spring, too. You see the worst case scenario/spring fever side of the horse right away, lol.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown