Just remember, humans are not horses, horses are not humans, try not to anthropomorphize?
What is comfortable for a horse on it’s four hooves and legs and mass above it may not be for a human on two feet.
Let the HORSE tell you what it prefers, not be determined to go by what we think it should prefer.
We had one large pen with an old concrete slab.
Horses loved to take naps standing on the concrete and then walked off it a few feet to lay down on the soft dirt for more napping.
Once rested, to just stand there, back to the slab they went.
It was clear that, in the whole large pen, that concrete felt best for them just standing there.
All I am saying is, don’t discount concrete flooring.
It is one more perfectly fine option, properly managed.
Just as compacted flooring is fine, properly managed.
Let it get uneven and it is not any more properly managed, something you won’t have happening with concrete flooring.
As for caliche, we have our own caliche pit, so that is why we use it for roads and tried for years to use it in horse stalls.
Driving on caliche when wet is like driving on toothpaste, it is slick and oozes around your tires.
We were continuously having to add flooring to our stalls, so much was picked up when cleaning stalls.
Now, if your horses won’t be in a stall that much, it probably will work just fine for much longer before it needs maintenance.
As I said, with mats, I would not know, we didn’t have those when we were training race horses on caliche floored stalls.
Everyone will manage their horses as they see best and either way will be fine, if it fits your horses.