Cast iron construction vs. aluminum, 5 stations vs 4, 100% clean primer disposal and priming at a different station than decapping vs Dillons decap and prime at the same station with associated grit and grime fouling primer slide every 500 or so rounds. Slightly cheaper caliber conversions. Strip or tube primer system vs tube. Bigger frame opening on RCBS than the 550. Supposedly faster caliber changes on the RCBS...
I have a 550 and am pretty happy with it . My problems with it are mostly the primer disposal and primer slide fouling problems, and the 4 die stations. I don't like aluminum presses but the dillon seems to last pretty well and the service is the best.
The advantages for the 550 to me are cheaper inital price, probably a higher resale price, much longer and proven track record, much better parts and accessories availability, and much, much, much better factory owners manual.
I compulsively bought a used but unused RCBS 2000 with a bunch of calibre conversions for a very good price. I guess once it arrives, I will run them both and decide which one is better for my uses. The other one will go on the equipment exchange.