When I bought my horse Faith, her breeder, Joe Goodenow, was a total mare lover. He said he preferred mares over geldings because they cared more and they would try harder.
I got that lol.. Both my horses were showjumpers and also both tb's. One mare, one gelding... That mare of mine, oh boy could she go, you would get into the showjumping ring and just absolutely fly through everything, you say speed round she says how fast kinda thing, she would just absolutely go for gold... My gelding... schooling at home he's a little speed demon when the heights get bigger, but as soon as you get into a ring, sure he'll jump everything, but it's gotta be at his own pace, no amount of leg, spurs, whip, nothing will get him going faster... We go into a speed round looking like a show hunter pair
(I saw "were" showjumpers, because my mare passed away a year and a half ago at 25years). Even at that age she still went for gold.. My gelding is 9, and a very very good ex-racer... and just doesnt try lol.
That being said I still don't really have a preference, even though mums skitzo mare broke my foot in 3 places 2 weeks ago