301 offers straight stitch only but fabulous for that as well it is more portable (lighter) and does have a top handle (handy). the 401/501 are essentially the same machine except for the retro styling on the 501. Same is true of the 403/503...difference being only straight-stitch without use of cams. The "zero" cam is for zig-zag and that one is hard to find. Be aware to look for the zero cam on those but otherwise the cams are not hard to find. There were 21 in total and the machines usually came with 8 (or 9?). I like the 403/503 for its simplicity...fewer dials, easier to use and less to go wrong. Most people favor the 401 or 501 with most of the stitches built-in but more available via cams. It sometimes does come down to availability. I was looking for a 403 when I found a 500 (rockateer) priced right in mint condition. I later did find an equally nice 403.