I have forgotten the differences I read about when I bought my 620. I like the saw and it cuts great. I have a 24" bar on it and use mainly for felling and bucking to a point. I use a smaller Echo for the limbing and stuff.
It is a heavy saw. If I try to use it much where I need to maneuver it a great deal my left shoulder (arthritis) will remind me to use the smaller saw.
I did buy mine at a dealer day and received a good percentage off that helped bring it down to what I wanted to put into it.
Oh, BTW these case for the 590 did work ok for the 620. The teeth that grip on the log (log dogs?) do rub the side of the case and it is a tight fit, but works.