Human manure is the worst. The chance of it containing pathogens. If you are using it in your garden.
Horse manure usually contains more seeds. Weed seeds.
If composted correctly any sh*t is no longer sh*t... And seeds are destroyed.
At the ranch we have a ton of horse crap... and chicken... mixed together and turned for years in a pile, makes about the same thing as down where the cattle are - There it is cow crap and chicken - our meat birds follow the cows kickin down the manure for seeds and larvae. But we collect and compost from the area around the water and where hay has been fed (where the manure is getting out of hand)... And we would pile up the winter, but they are gonna go to slaughter before then. Cow based compost is richer, and takes less time to make. Horse takes years in the pile, even helped along with chicken... So cow is preferred.
As for humanure, there is debate in that (prions, and ONLY prions if well composted) and I fall on the side of caution. Once I am up in the sticks, I will be composting my waste... That happens for 2 years with shavings and straw to aid the breakdown... When that is handled it will go in the fields... I will have enough cow, goat/sheep, and chicken poop to make due without it.