I have a SDB and a 550. The issue I had with the SDB is leverage. For the press to cycle with any ease you have to lubricate the casings. You also have to give it a little shove to seat the primer, which is not normally a problem unless you have shoulder problems. I have loaded thousands of 45 and 9mm on the SDB. The dies on the SDB are easier to adjust. You just turn the die with a wrench. There is no lock ring involved.
I purchased the 550 because it has much better leverage for cycling the press and primer seating is much easier. I still lube the casings although I would not have to. It makes the press run smoother. The 550 has more room on the shell plate. It makes it easier to handle the cases if needed. The lack of auto indexing is not a problem, as mentioned before it is a plus when clearing a problem.