I'm new to reloading and bought one in .40S&W.
I looked at the 550 and the 650,but reality dictated finding a used square deal.
I am not a rifle shooter,so for me a used SDB,was a no brainer.
Dillon will send you the parts you need(for free) if something wears out or needs refurbishing.
You can find them used in the $250 to $350 range.
I added a strong mount and bullet tray.
I also have a quick change setup for 38sp and 357.
My friend bought a used SDB in 45acp and he likes his too.
Set them up and start cranking out ammo.
The one difference I see between pre B and B is the failsafe powder drop mechanism.
The new B has a rod that actuates the powder measure through two levers,(supposedly more failsafe) ,the pre B works on one lever and springs.
No big deal either way,
When going from 45acp to 9mm you will also have to change out the primer feed tube on the machine.Large primer to small primer,unless you are using Winchester small primer 45acp.
My friend just bought a 9mm SDB and has two set up,one for 45 and one for 9mm.
Never has to change anything.
I'd have no problem in recommending an SDB.
I also like the fact that it auto indexes.
Find a decent used one and start rolling out ammo.
You'll wonder why you didn,t get a progressive a long time ago.