I have one of each. The 650 is dedicated to one caliber (with a case feeder). After 500,000+ rounds it is getting a bit wobbly. Dillon has offered to over-haul it for free.
The 550 has a bunch of tool heads pre-loaded with the dies. I use it for the calbers I only load a couple thousand at a time.
I have not had good luck with priming cases with these presses. The 650 only loads vigin primed cases. The 550 loads cases i have sized on another press and primed on my Lee Auto Prime.
I like the Dillon equipment and found them good peple to deal with. That said, I have had excellent service from Lee, RCBS, etc.
An electicien working on my fuse panel dropped his cirgarette into my Dillon 550 powder thrower. Apparently the 2 pounds of 231 made quite the Roman candle..... I mailed the melted blob of plastic and pot metal back to Dillon with a note that it was "eratic'. They replaced it without commnet or charge. We are blessed with outstanding suppliers.