In general I think mares are tidier, but there
are always exceptions and many people will know of one. ;-> The
geldings of my acquaintance are not particular and will drop a load wherever
they happen to be standing. The gelding I have now will pee on his own
hay but he is 23 yrs. old and we chalk it up to senile moments. Peeing
on the mare's hay pile is probably more
deliberate.
Feral horse herds will usually choose
one spot for the potty area so it doesn't foul the rest of the
grazing. They don't linger long in one spot and keep moving throughout
their territory. When they re-visit that spot later on the grazing should
still be good.
With large enough turnouts and pastures you may
note the same thing. I'd guess the gelding you sold was the messy
one.