Mares, like most females, are too temperamental (well I know I am lol).
Geldings, like most males, respond to basic things, food and a commanding female voice lol
I had a gelding first, and always thought, geldings more reliable less moody etc etc and the mares I knew were moody too!
But then I bought a filly and totally changed my mind. She and I have so much closer a relationship after 2 years than my gelding and I have after 9. And she doesnt get moody moments either. AND she loves having her lady bits cleaned which is totally unlike Lethal Weapon cleaning attempts on his majesty - nightmare!
So mares for me every time now...
I prefer mares. Geldings always seem to try to be boss as if they have to make a point. I can communicate much more easily with mares. My gelding also seems to be jealous of my mare getting my attention, so he is always having a go at her when I am about, although I've watched from a distance and if I'm not there they get on fine.