I never liked the Arabian...very fast and agile but chucks you off when the wind blows through the trees or a deer farts...very skittish. The Turkoman being a crossbred warhorse/Thoroughbred is as fast as the Arabian but a great deal more stable, it's more likely to stamp on a snake instead of throwing you off a cliff leaping about like a rabid poodle.
I like how the different horse breeds have their own personality traits too, makes you more likely to actually like your horse. The Turkoman also one of the tallest horses in the game, less like strapping a saddle on a dog with hooves