I am not sure on which of your products would be the best for me. The changer would be mounted in my Race Trailer, but am not sure on which one to choose and I want something that will last and perform well. I am not sure what the differences are between the models. Any suggestions?

Yes.
The differences are:
  The ProModel center post does not need to index the center of the wheels. It is easier to use with only a "T" pin to pull to release the arm to hinge aside. It self locks when moved back into position. This model is extra heavy duty, and meant to be stationary, although some of our customers to take it on the road with them.
  The Jr. Pro Model has more adjustments but is portable. Personally, I would rather mount a Hitch Receiver outside the trailer (in the wheel well, or where ever it can be welded on) and plug the changer in on the side, using our HitchMount and the Classic Model, or the Jr. Pro Model. It is far too congested inside most race trailers, especially when it rains. Most people have pull off awnings or butt an “Easy Up” portable tent next to the smaller trailers anyway so you would be covered in he rain.
Actually the Classic Model will do what you need unless you are doing Scooter and ATV wheels less than 10" or truck tires over 31" Diameter. Trailer and automotive tires will usually fit on the classic. The Classic Model may be dwarfed by the other two but is still a hefty machine. The quality and strength are all the same. Just the range, and the articulation of the machine changes. The Classic Model, when mounted to the floor, needs to be slightly farther away from the wall behind it compared to the Jr. Pro Model.