My experience is that donkeys are smarter and more personable than horses; for ruminants, they aren't nearly as stupid as some, like sheep or cows. Then again, they ain't gonna outsmart many mammals, either, except maybe the great cats, who are almost supernaturally stupid.
So if you like affable, semi-bright, fairly low maintenance critters, donkeys are a good choice. Also, for some reason, like horses, they seem to get along with dogs real well - always thought that was weird, dogs being predators and all. A buddy who ran a stable had a big, mean, moronic, stallion as a tenant - damned dangerous critter - we'd put my friend's large golden retriever in with the stallion and it would calm him down. Still couldn't trust that horse, but he quieted down enough you could tend to him. Carefully.