Remeber that for the trot, it's the "diagonal" and for the canter , it's the "lead" that you must take into account when doing circles. When going on a straightaway, trot on whatever diagonal feels good to you.
You may want to trot on the trails and after you have picked up the diagoanl that you natural feel is best, look down and see which one it is. Do this many times and see if your horse isn't kind of "putting" you on the same diagonal all the time. Often one diagonal will feel better than the other.
The reason for positng to the outside diagoanl on a circle is that when we post on , say the left diagonal, (we are circling to the right), we wil come out of the saddle as the outside REAR leg is thrusting backward (impelling the horse forward). That thrust is what lifts us off the saddle. Obviously, both legs thrust with each stride, and we coudl be lifted by either one, but since the body of the horse on a circle will be longer on the outside, shorter on the inside due to the bend of the circle arc, the thrust on the outside will need to be bigger and with a freeer reach forward of the front leg. THUS , coimg up and off the hrose when the outside is doing the work is better for the horse!