I have used both over the years, and to me it is a tough decision, I am using a 550 presently.
The SDB is a nice compact machine that auto indexes, which makes a double charge next to impossible. The only downside to it in my opinion is that you MUST use Dillon die sets with it and it will not reload rifle.
The 550 takes standard dies so you can use Hornady, Lee, RCBS etc dies in it and it will load rifle cartridges. It does not auto index so if you forget to manually rotate the shell plate and stroke the handle you will have a double charge.
With my 550, I can accomplish a caliber change including changing primer size in about 5 minutes.
My father has a pair of SDB's and one is set up for small pistol primers the other for large, this greatly speeds up caliber changes as all you have to do is swap die heads and shell plates.
Having said all this if I saw a good deal on a used SDB or 2, I would probably scoop them up and use them exclusively for my handgun cals.
Steve