Both zigzag without needing a removable cam, both have a mechanism to lift the feed dog plate for darning, etc. My recollection is the 603 has the Touch&Sew pancake bobbin and the 401 taking a class 66. They both use the same needle. If you want built in special patterns go with the 401, if you're straight stitching and zigzagging, either is fine.
My bias would be towards the 401, only because of problems with other Touch&Sew machines, which isn't fair to the 603.