The 319 is most people's favorite of the swing needed machines because of its built-in stitches. I think you can do the same stitches on 206 and 306, but cams are required for all. The 319, you can do certain stitches using the levers, plus you have the cam capability as well.
319 manual
http://www.users.on.net/~instawell/singer_319manual.pdf