With either of these choices there are still a couple of problems with horses that should be considered and will have an impact on which direction we go toward horses.
1. Eventually everyone will have a horse, as the value of them will slowly decrease at first, with a rapid fall at some point when overpopulation occurs. No matter which choice is made, unless there is a method for killing off old horses the population will increase exponentially.
2. The roads we have now aren't suitable for horse travel, All the interesting turns and twists, ups and downs we made to make awesome looking roads fail miserably for horse travel. Constantly crashing into trees with low branches, or trying to navigate the crazy beasts down narrow streets and roads.
I have an alternate solution to help control the population. Don't allow horse breeding at all. I know its a bit radical and will upset those who have spent a great deal of time playing as horse breeders. "Even if we didn't have horses in game at the time". If the new wilds is to be a super dangerous place that no one should enter without careful consideration, there should be something valuable to seek after. Finding rare horses in the wilds would be difficult, and surviving the wilds with your new found horse would likewise be difficult.
Perhaps the establishment of horse ranches, and small castles will need to be built to protect them while they are being "broken in" so they can be transported back to the more civilized worlds. This would allow the breeders an occupation still, while making it something more interesting than simply breeding.
It would also help control the population. As horses would be found instead of bred. And the likelihood of a tamed horse and rider dieing on the journey back to the capitals would be greatly increased.