i have owned a 650, and currently own a 550b and a square deal (auto indexing for pistol calibers).
the auto indexing on the 650 is nice...right up until you have to stop because of the occasional issue. i like that the primer feed system is at a different station which keeps it free of debris from primers getting knocked out. what i DON"T like is that the primer feed system keeps spitting them out should you have to stop and re-start...which wastes primers and grinds on me (i hate wasting components).
the 550b and the square deal, you can just not set the new primer in the case and the feed system doesnt keep cycling primers out. add the additional cost of the 650 and toolheads, and the 550 is the one to have from dillon.
the bottom line is they are all very good presses. the 550 will do what you need to do unless you are looking to load 50bmg.