Go big early. I've never regretted it or heard myself say "I really didn't need this big fancy thing." Also, hearing someone say "caliber changes take longer" is like hearing someone say "the microwave isn't heating my food fast enough." At this point, it can't get much faster! Caliber changes for me take about 5 minutes on my 650 if I don't have to swap out the primer system, and maybe 10 if I do. Now, I've invested in the spare toolheads so I'm not having to re-set each die, but that would be an issue in either system.

Go 650 first, and then get a 550 to play with if you still think you need one.