That would depend on how many calibres you intend to reload for and whether or not you intend to run a case feeder.
If you are reloading a moderate volume of several calibres, you are better served by the 550, as it is simpler and less expensive to change calibres. Auto index doesn't make much difference unless you have a case feeder. I can get a cyclic rate of around 400 rounds/hour with my 550 without a case feeder.
Where the 650 really shines is if you wants LOTS of ammo and you don't plan on changing calibre often. You need the case feeder to really take advantage of this press. The downside is that it costs more and is more complex to change calibres.
When it comes to reloading equipment, there is nearly always a tradeoff between flexibility and rate of production.
Some items you will need for a Dillon progressive press:
-Reloading manual
-Calipers
-Powder scale
-Bullet puller
-Primer flip tray
-Calibre change kit for each calibre (shellplate, locator buttons, and powder funnel/case mouth expander)
-Toolheads and powder dies - You don't want to have to set up your dies every time you load a different calibre
-Case trimmer for bottleneck cases (I use the Lee setup with a drill)
-Case lube for bottleneck cases
-Strong Mount