To me, it's not about the gender, but about the personality. I've met many mares and geldings, and I honestly can't remember one horse that stood out as 'OMG you are so moody/grumpy/etc I can't stand you'.
All horses are different.
I've met a mare that only likes one other horse, and absolutely hates any others. That doesn't mean she can't be ridden in a group or trailered with other horses. She was taught to behave. It might be obvious that she's not too happy about a certain horse being too close, but she doesn't bite or kick. She behaves.
I love my mare's personality. She's loves everybody, horses and humans. Tolerates any horse near her. You would never know if she's in heat unless she's with her favorite gelding.
I've had geldings that were so smart, curious, and loving. They were like big dogs, plus they would chew on and play with every bucket, open any door, untie any lead rope, escape every paddock! But I loved that they would go against the norm, and try to open/untie/whatever things for sometimes an hour while their friends were like, "Whatever, that's too much work. I'm eating grass. Nom. Nom."
They all have a bit of the happy go lucky/in your pocket personality, and I love that kind. Whenever I find a horse like that I instantly fall in love, mare or gelding.