There is the trailer ride to consider, but also the stress of staying overnight in another location. Some horses are very used to it and don’t seem bothered. But some stay anxious that they are not at their own home with their own herd.
I watch for signs of how the horse handles staying away from home. Are they constantly looking around for danger or their herd, or relaxed and eating hay? First thing in the morning, does their tail have shavings in it, showing they did lie down to rest? Are they eating and drinking relatively normally, or too worried about their surroundings to eat & drink as usual? That kind of thing.
Also, if you have the opportunity to use a camera to watch horses in a trailer from the towing vehicle, every mile isn’t necessarily a slow walk. That is mostly for roads that aren’t smooth (and some highways are not). On a smooth highway (interstate) at a constant speed, many horses will sleep or nap.
Also horses do stand up most of the time, and if outside they wander while grazing. So for them standing in a trailer may be somewhat like us sitting in an airline seat on a plane in flight, or perhaps a subway for a trip with more motion - they are in the posture they hold for most of the day, anyway, but on a moving container. Some people are more comfortable in an airline seat or on a subway than others. The same for horses during trailer travel. Bumpy or uneven roads are like bumpy weather for us on an airplane, much less comfortable and requires the horse to move to keep their balance. The trailer speeding up and slowing down is the other main source of motion activity for the horse - kind of like the subway on curves and making stops & starts.
However, there are some horses that stand braced for the entire trip, no matter what. They definitely show that they feel it after the trip. I wish I knew better how to help them relax without doping them just before a competition.
Where I used to live, most of the events were less than an hour from the barn. The horse and I were both better off to drive home on Saturday night and let him be in his own stall overnight, and then drive back on Sunday morning. However, if an event is 2+ hours away, I tend to re-evaluate what plan will be the least stress on the horse and on me. I don’t mind getting up at Oh-Dark-Thirty to go to a clinic or event to compete that day. So the decision has more to do with whatever is the kindest to the horse & self, taking all things into account (the roads, stabling, accommodations, ride times, etc.).
I haven’t been to an event that didn’t wind up by 4 pm on Sunday, leaving time to get back home at a reasonable hour even if it is a bit of a drive. It would have to be a long trip that I would not come home on Sunday evening. Even if it were 3-4 hours drive, I think my horse would vote to go home and be back with his herd in his own place that night. However I would also factor in how tired I was feeling vs. the length of the drive.
(Back in high school / early college I never, ever stayed overnight. On show morning we typically drove out of the gate at 3:30 am or 4 am. Self & my horse, plus whatever other rider and her horse who caught a ride with us. I recall telling one that was dithering about leaving so early, that the trailer was rolling out at that time, whether she had her horse loaded on it or not. :winkgrin: Then one day I became an adult and realized that going the day before and staying overnight was a way better option for both the horse & me than leaving at 3 or 4 in the morning and getting home close to midnight, almost every weekend. :lol: )