IMNSHO,
The machine a person should buy at first, is the machine they will end up with 20 years down the road.
I honestly believe that for most pholks that like progressive reloaders, that "eventual" machine will be a Dillon 650 or 1050.
Having said that, I still think the Dillon 550 and the Hornady and RCBS are truly great machines. I just think the 650/1050 are better machines
I still have, and use, an old Dillon 450 that has been partially upgraded, for those calibres that I don't load in great volume, such as .30-30, .44 Magnum and .45 Colt. It's a great little machine, but it just wouldn't cut it for loading 9MM, .40, .45ACP, .223 etc., in the volumes that I load.