When I was younger, I prefered geldings because they were level headed and easy to work with.
While training fox hunters and polo ponies, I noticed the fox hunters had an assortment of geldings and mares, while the polo team had strictly mares with only one gelding. I asked why, and they believe that mares were more competitive and had the "get out of the way or I'm going to slam you" attitude that they needed during polo competitions. I tallyed the way the mares played against the geldings that we had for polo, and it was true. The geldings (one in our team, a couple in some others, some didn't have any at all) had a gentle, easy going nature (and very trustworthy) while the rest of the horses were mares and a lot more competitive in that aggressive sport (though slightly less trustworthy). They'd SLAM into eachother, chase eachother down and put that mare-ish attitude to work. The geldings didn't like to slam, they'd chase with half as much heart, but they did their job well and honestly.
I don't mind either one. I've worked with geldings and mares both equally and found they both had strengths in different areas. Polo obviously isn't the standard for judging gender, but it seemed to be true for the players, anyways.
I've seen some lazy, lazy mares and I've seen some driven geldings, so its not 100% accurate. I think that gender really doesn't matter if you find a horse you click with. If you narrow it down to gender, you might miss out on your perfect horse. I never thought in a million years that I'd end up with a mare. But I did and she's awesome.