I have owned both the 450 and 550 model Dillon's and they were mounted side by side on the loading bench. I prefer the 450 to the 550 if you add the new priming system, new powder measure, and the kick out and thumb index pieces to the 450. I would rather change dies and have them lock down dead solidas on the 450 than change heads that don't lock down completely solid as on the 550 but that's justs me. When I'm reloading, I like everything to lock down as solid as I can make it. I use a RCBS Rockchuck press for my rifle loading anyway. I handweigh on a electronic scale all my rifle powder charges because I don't completely trust Powder measure when I get over 45000 lbs of pressure in front of my face. Just my preferences and I suspect there are many who don't see things as I do and that's ok. James