Mustang premium or "The Boss" is going to be very hard to find let alone price. I talked with a well known distributor this week about the Boss. They said only 99 were made ( I know a few have reported name plate numbers slightly above 100), and this may be the most limited number of a model Stern has made. The distributor was sorry they did not order more and that their advise to me was to get a real good price if I ever sold mine.
I have played the Pro, and I believe the Premium is worth more from a play perspective. That of course is a matter of opinion and always seems to get debated in a fierce manner. Best of luck, both machines have great light shows and are fun to play.