To start with, one must realize that the world has, and continues to have, horse-sized chickens. Once upon a time they were called dinosaurs. The larger members of the Dromeosaur family were, in fact, roughly the size of horses. If you believe you are able to take on a Utahraptor or an Astruoraptor and win, why then yes, fight the horse-sized chicken.
The modern version of horse-sized chickens are emus, cassowaries, and ostritches, all three of them well-known for being able to kick a person to death. If you think you can fight an emu, then yes, by all means, fight the horse-sized chicken. Keeping in mind that the Australian Army could not defeat a small contingent of them.
Now on the other hand, fighting a hundred chicken-sized horses has more unanswered questions. What is the actual mass of these horses? Are they coordinated, or are they more likely to run away like regular horses? Are they as fragile as regular horses? Regular horses are fragile because they have a great amount of mass set atop very flimsy legs. If they were the size of chickens, would they retain this fragility, or not? It might be possible to obliterate such a horse with one kick, or perhaps not.
I think I would pick the horse-sized chicken simply because I would know exactly what I was in for and I would be able to plan accordingly. The tiny horses, who knows how they work?