Found my answers. 600 has a built in cam stack + removable cams, 603 uses cams only - no built in cam stack. It's just like the 500 and 503. Otherwise, they are the same machine. Bobbins are supposed to be cranky on both.
I had a 778 (788?) that I donated to the school, and never had a problem with the bobbin winder. The kids are struggling with it at times. I guess I was just lucky.
Thanks,
bkay