They are both very good.
I have two 550's and had a SDB, just sold it a couple of months ago when I picked up the second 550. A lot of the pro's and con's have been posted. The auto indexing of the SDB is nice but I prefer the 550 I have total control. The 550 can be used in ways the SDB...
Great deal
I would say buy it asap. If it has 4 setups with tool head, powder measure and dies; you found a great deal. If you want to part with a tool head & powder measure setup or two let me know.
This is a great deal.