Both:
Dogs - Better, longer & faster pounding; constant cumming; knot & being knotted up; better tasting cum; best for quickies and can repeat. Plus you can just have a good, quick fuck with a random dog you encounter!
Horse - Bigger, sexier looking; smooth feeling penis skin; better to suck on and have a dick head to suck on; horsey smell; strong cumblast & lots of it; fills you up completely when inside you; and it just looks so fucking sexy when hanging there!
I've experienced both and cannot choose. ? ?