All of the Dillon presses are good machines...but hopefully you understand the differences.
SDB - handgun ammo only. It uses a proprietary die system. It auto indexes. Good press / no case feeder option - and no powder check die option.
550 - progressive / but it manually indexes. No auto index / no powder check die available.
650 - progressive, auto indexing, has room for powder check die - runs best with a case feeder...
1050 ...is a commercial machine, does not have the no bs warranty...but its a good machine / but way more than any casual shooter would probably need.
deal breaker to me, on handgun ammo especially - because most of us want to run relatively high volumes - is to have the powder check ( so the SDB and 550 are both out ). 550 indexes manually ...to me its a pain in the butt...I'd go with the SDB over the 550 if all I wanted to load was handgun ammo. 550 has some oddball rifle calibers that the 650 does not have...so it has its place...but the 650 is a better press.