Because I would always want to sleep with my partners, without risk of the bed breaking or being squished dead when they roll over, dogs for sure.
If I have to leave them (horses) out in another building or another room of the house at night, that is just too inconvenient to have the same kind of bond. It's a different kind of bond because of that. Also, it's no fun being stepped on, and the lifetime upkeep, including veterinary, farrier, and food, is just astronomical compared to partners of other species besides maybe cows.
Dogs for sure.