Saying that though I have had 1 really sweet ottb mare who was so calm even when in heat that no one would know enless they looked at her charts.
As for geldings I have seen some moody geldings usually rare but it will happen :)
At the point I am at, I would rather just get geldings from here on out. After my current pasture puff mare passes she will be the last mare I have for a while. I think the herd dynamics will be alot better without the mare and just the boys. But I wouldnt rule them out, some mares are great like my ottb mare. She was a dime a dozen with her no attitude lifes style.