It can be a tough call. I’ve done both, and here’s what I learned:
Trailering in to a place works only if
-you are able to work your horse consistently at home so that your horse is fit and able to benefit from the trailer-in lessons
-your horse is trained/calm enough to load, tie/stand for tacking/mounting and doesn’t require a long period to longe/settle in first.
-you are a regular customer so that the trainer is able to build familiarity and rapport with you and your horse
My current horse can be pulled from his field or stall, in any state of work or vacation, and go on the trailer, and get off at the destination and not be an idiot. My last horse was much older and had hauled extensively, but never did learn that trick. They just settle well or they don’t. Sometimes no amount of rehearsing it will get them there. Not every place is condusive to letting you longe/hand walk your horse for 30-45 minutes before your lesson because your horse is a whacko. I’ve also been to places that were seemed welcoming enough, but clear preference and superior lessons/attention were given to boarding students. I can’t say that I blame the professionals there. The person who is paying you full training board should get more than the person who is paying you a single lesson and haul-in fee.
Keeping a horse at the trainer’s has always allowed me to get the most riding done. But it can be very difficult to find a place that is a good fit.