dont know from exper. but from what i can tell it is going to depend on your higth and weight as to which ones to get. the longer ones set the ground clearance higher which aids in not scraping pegs in the twisties.
Yes, 12.5 inches are the stock length. Whether you need the HD springs though depends on your stock. It's generally the bigger guys (200+ pounds) that need the HD springs. My standard Progressives are fine for me. I got the 13 inchers for better cornering performance but I think I should've went even higher.