I have 2 550', a 650 dedicated to 45 ACP and a 1050 dedicated to 9mm. Never used to be worth it to load 9mm before but that changed. The 650 and up are not near as easy to convert calibers on. I have several 550 toolheads set for all the calibers I use and can switch them in about 5 minutes. Very easy once you've done it a few times.
So if I only could have 1, well 2 as I believe 1 for small and 1 for large primers, the 550 wins for me.