I like both really...and I'm not sure that I would choose one over the other.
I have two mares and one boy right now, and to be honest I prefer working with the mares, because they aren't so lazy that you can't tell they are learning.
I know that mares can get snotty around their heat cycle, but to be honest, so do I, so I don't really use it as an excuse, more so that I understand what they might be feeling! I've never had a really exceptionally ****y one though, which I consider to be a blessing! Haha...I'd still like working with them though, because I find I enjoy that extra fire a mare can have, over a gelding.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."