I have a rainbow of colors on my reloading bench. Though my progressive press is a Hornady, I also have a couple pieces of Dillon equipment. In one of my freshman engineering classes at college a zillion years ago, the point was driven home to me that every design is a compromise between multiple factors. The designer decides which factors are the most important and then creates a design that favors those factors at the expense (do one degree or another) of the other factors.
Each model Dillon press is clearly designed to favor a different set of factors. For example, the 650 is designed for people who will be pumping out 1500-2000 rounds of a given caliber at a time. The ease of caliber changes is less important.
The Dillon 550 on the other hand is designed to allow easier and less expensive caliber changes at the expense of other factors.
Hornady, RCBS, Lee, etc are no different. And is designed to meet a slightly different set of needs.
Me? One of the reasons I decided against the 550 (or any 4-stage press) is that if I decide to move to separate seating and crimping dies I would be SOL. I also felt that Hornady did a better job on the cost-vs-time tradeoff regarding caliber changes.
My advice would be to look at them all, understand the pros and cons of each, and then make a decision based upon what is important to you. BTW, when I was researching progressive presses, I recorded my notes and observations here:
http://www.shootandreload.com/category/choosing-a-progressive-press/
And though it deals with the Dillon 650, you may find this helpful too. A guy owned the Dillon 650, Hornady, and Lee Loadmaster for a year and used them side-by-side. He recorded his observations here:
http://www.comrace.ca/cmfiles/dillonLeeHornadyComparison.pdf