Nothing wrong will Dillon dies in the SDB. Nothing at all. And caliber conversions for the SDB are no worse than for the 550/650 - when you add up all the parts.As stated, the SDB uses proprietary dies. So if you go that way you are locked into those Dillon dies for the life of that press.
Many people eventually wind up with multiple (progressive) presses dedicated to a caliber. If that turns out to be your future - or if you want to use dies from another manufacturer - the SDB you buy now could eventually be your (eg) 45ACP press.
If those two outcomes sound appealing, go with the SDB. If not, go with the 550.
There are really only two shortcomings with the SDB
- The amount of room around the shell plate is limited, which can be a problem for guys with banana-fingers lacking dexterity.
- The amount of press mechanical advantage is limited and resizing a 44 mag case is best done with lube. And you can really feel the different primer types. My point is that you shouldn't hold your breath waiting for a 460 S&W caliber conversion as that is a bit too much (height and force) for the SDB.