I didn't see this addressed in any of the other answers, but essentially it boils down to:
Whatever is most cost effective for the job
As @PavelJanicek gets at, horses will always be cheaper.
Horses aren't very clever and are not very good at adapting to rapidly changing work conditions. They're good for jobs where an increased amount of brute force has a linear increase in work produced.
Humans on the other hand, augmented, magical, or otherwise, can handle rapidly shifting conditions where the "goal" is an abstract concept and which flexibility and creativity is more important than sheer power.
You know that thing about monkeys and typewriters? Well, we don't employ an infinite number of monkeys because while individually they're paid peanuts, and the produced result is quantitatively very high, the quality is rather poor: it takes almost as much work from an intelligent human to sift through the crap (pardon the pun) in order to find those works of Shakespeare than it would be to just employ the human in writing it in the first place. As an added benefit, you save the cost of an infinite number of peanuts.
Your augmented humans would be used in scenarios where their brains are more important than their brawn: the ability to identify strangers, perceive threats and dangers, or perform delicate manipulation (even with the strength of a horse, human fingers are a tad more nimble than hooves).
Search and rescue would be a good example: collapsed buildings, avalanche zones, etc. but by no means the only place. Factory machinine might also be a good place. Lots of strength, requiring less energy keeping the forges hot (because the guy can just bend metal bars with his bare hands) along with all of the creative ingenuity the human mind can bring to bear.
But as a general rule, any job that can be performed better by merely increasing the employee's strength will go to the horse.
That's why even though in the real world we've replaced horses and humans with increasingly complex machines, but we've not had the unemployment crisis that laying off all of those unskilled workers would logically represent: instead they find jobs that the machines can't yet do such as keeping the machines oiled.