I think what you are referring to as a step down regulator is actually a linear regulator. Basically, it reduces voltage through resistance, hence the reason it generates heat. As you can imagine, that heat is what makes them inefficient.
I don't have any experience using a linear regulator for that much amperage but if I had to guess, it is going to get HOT. Like fry-an-egg hot.
A switching regulator will be more efficent and run much cooler. If it were me, I'd go with the switching regulator, even if you need two.
Are these for USB hubs in your install?