I bought my SDB back when they were $150 as well. Loaded on and off for decades. Recently came into a 550 that I had rebuilt by Dillon. I have to say that I think I prefer the manual indexing of the 550 over the auto indexing of the SDB. Speed was never much of a concern for me...either machine will crank them out faster than I can shoot them.
On the SDB, something would get mucked up (usually crappy brass at stage 1 that I would have tossed if I spent more time examining brass) and that would interrupt my flow and cause me to clear the press and re-examine.
The 550 gives me a better feeling of control when something gets mucked up resulting in smoother operation for me.