I don’t have any preference, but I think a horse trailer it’s more convenient. With my experience, a horse climb more easily in norse trailer, because it’s longer and the horses dare to go there
And I think it really depends on how often you use it as well. I had to take a trailer test. But decided trailer was better for me as I needed a car anyway. But trailer costs me nothing sitting there, with a box you have the cost of 2 vehicles to go wrong.
I hope this helps.