One thing to remember when choosing a hauler is that of course not every client will be happy. There are always those that are impossible to please I am sure we all know of a few people like that
Things to do are to check references. If they leave numbers or e-mails on their site check them. Not only current ones but go back a year or so even more if available. If someone from a year ago still remembers it as being a good experience chances are it is.
If you hear one bad and 30 good chances are the good are more true to the company then the one bad.
Make sure the trailers have plenty of vents and windows. Make sure they have dividers that go to the floor as opposed to make shift pens which can be very dangerous.
The things I look for from references are first and foremost was the horse in good condition when it arrived. I account for some horses stressing a bit and becoming a bit sucked up that is somewhat normal especially if traveling to a different climate.
Was the horse calm around the driver/handler or were they skittish when normally not so. Was the trailer clean and other horses looking good. * I am always one to peek in and check out all the horses on board*
Was communication good both for the seller and the new buyer?
These are the things that I check for.