Jim,
I bought a 650 when they were first introduced and eventually bought another one when I started shooting pistol again.
The pros and cons have already been discussed so I'll simply say that if you buy a 650, it's doubtful that you'll ever wonder if you should have bought a 550. However, if you buy a 550, it's likely that you'll second guess yourself regarding a 650.
As expected, opinions are all over the place. No wonder we can't all agree on who should run our country.
In a nutshell, it sounds like you really can't go wrong with ANY Dillon press.