Strong Mount. The bullet holder attached to the strong mount is a very nice addition.
If you are even thinking about loading both large and small primers, getting two is 550’s works much better. Its not impossible to change out the primer system but not having two makes the entire process much quicker and less worrisome.
Next thing to consider is sticker shock. Have a hard time finding a more versatile reloading system. Getting there requires big bucks. Over $200 per complete caliber set up at current prices. (including tool head, powder throw and caliber conversion but not dies.) I have right at ten that I have collected through the years. Scares me as to how much they now cost.
I use my large primer 550 to load LRP 6.5 CM rounds. First prime and size, remove case from shellplate put casein case block next to RCBS manual powder throw, throw charge and trickle to weight, pour powder into case. Move case back to shellplate and advance two spaces, put new case in shellplate under sizing die, put bullet in charged case, Size/prime case and seat bullet. Time saver is doing two operations at same time.
I have actually loaded using the Dillon Powder throw. It works, accuracy was degraded slightly (might have been me) but I hate chasing powder charges, and throwing a charge then weighting and trickle to exact weight leads to more confidence.
For my contest rifle, using small rifle primer cases I use both of my single stage presses, one to size one to seat, and use an RCBS primer tool to prime cases. But I also double clean these cases, once before I size and again after sizing to remove lube. Could utilize my small primer 550, but since I have the single stage presses, and it would cost to purchase the shell plates/tool heads/stands/ or be darned inconvenient to move everything needed from one press to another, I just use the single stage.
When I shot IHMSA. My main money gun was a custom XP-100 in 7 TCU. Ran it straight on the Dillon. Checked charge on every fifth case, shot a whole bunch of perfect scores. Never underestimate a plain old Dillion 550.