For a few years I had both a 550 and a 650, so I can compare.
The 650 was great for cranking out lots and lots of the same thing. I primarily used it for .45ACP ammo with 200gr LSWCs. I could crank out a few thousand rounds of pistol ammo on a Saturday morning, but then the machine sat largely idle.....sometimes for months. I had 9mm and .223 conversions for my 650, as well, but I didn't like doing the conversions.
The 550 really shines with quicker/simpler change-overs, and doing smaller runs of lots of different stuff. Even though I had the 650 caliber conversions for 9mm and .223, I loaded most of these on my 550, so I didn't have to convert the 650.
The 650 is a great machine, and I really enjoyed loading on it, but I just didn't think I shot enough to justify keeping it. I eventually sold the 650, and now I have a pair of 550s. I leave one set up for small primers and the other for large primers, and this seems to suit me better.
