Have you tried either machine? If the 401 is working, I would choose that one. They have such close tolerances that they tend to freeze up if left unused for a very long time. It takes patience, lots of solvents, etc. to get them unstuck. If it's not stuck, you don't have to worry about that.
The 401 was sold, that the time, as the best sewing machine Singer ever made. The only downside I am aware of is that it doesn't drop it's feed dogs for quilting. It has complete built-in zig-zag, so you don't need cams
I'm not as familiar with the 603 as the 401. It should be a good machine as well. I don't believe it has built-in cams for zig-zag.
bkay