I have 2 550Bs - one for small pistol and one for large pistol. It saves a lot of time.
I considered a 650 as a second press and decided against it. Why? As you mentioned, parts interchange readily and I prefer manual indexing anyway. I don't do over 200 - 300 rounds per press per session either.
Two of my friends have 650s and they are happy with them. Still, they like the route I took with the two 550s.
For me, the 650 is more press than I need and I shoot 1000 rounds per week of different calibers. Of course, YMMV.