What do you have and what do you prefer?
Trailer would be cheaper as we already have a 4x4, however I could only tow one due to the towing capacity of the car. Cheaper to maintain and no extra tax etc.
3.5 horsebox is another option which I would feel much better driving, but will it cost me a small fortune to run?
I would be using it a few times a month, one a week perhaps in summer!