I've really gotten into the technical aspects of riding since I read a fascinating book by a career diplomat named Noel Jackson,
Effective Horsemanship. I think it's out of print, but I managed to find a copy to give to my trainer for Christmas!
Jackson was posted to Portugal and was able to train with the military riding school at Mafra. He is a big advocate of physical mechanics and dressage in everything from jumping to making polo ponies. It's a very good book!
I thought about it and I don't notice any real difference at all between Bay's trotting or the comfort of posting in either diagonal. I think he must be pretty even.