You asked about Lee dies with a 550:
I use a Lee U die for both 40 and 38 Super. I cannot use the lock ring on the top or the bottom, so I just drilled and tapped the toolhead for a small set screw, something like 8-32 or so, works fine. The U die may have a shorter thread span than other Lee dies, I don't know. With the 650, you are able to put the lock ring on the bottom.
The nice thing about the Dillon dies is that when loading lead, you are able to remove the internals of the die for cleaning without disturbing your seater or crimper setting. As before, I use the Lee U dies in the first station, but Dillon for the remainder. For 45, it's all Dillon.
Hope this helps.
Rich in VA