I habe bought fruit trees from both Miller and Stark Brothers. The Stark Brothers Supreme trees fruited a year earlier than the trees that I bought from Miller, and also the non "Supreme" trees that I bought from Stark Brothers.
Typically a dwarf will be 20%-25% the size of a "standard" fruit tree, semi-dwarf will be 40%-75% the size of a "standard" tree. I planted all semi-dwarf trees, with a few standards because they weren't available in semi-dwarf size. Here's some reading material. Pay attention to how far apart you plant the trees. At first you'll say "no way, that's too far apart" but after 5 years you'll say "I'm glad that I didn't plant them any closer together."
http://growingwithstarkbros.com/?p=2692