Originally Posted by BoJangles

Marie, it depends on what you are doing with the machines. I would take the 15 over the 66 simply because it drops its feed dogs - the 66 does not drop the feed dogs - for free motion quilting.

Here are a few other points:

The class 15 bobbin is slightly larger and holds more thread. The 15-91s have the bobbin going in vertically, not drop in horizontally like the 66's. This can be helpful, especially in FMQ, because the thread path doesn't need to take that 90 degree turn as it sews (the 66's, and any horizontal bobbin machine... the thread turns a 90 before creating the stitch). It's a tension thing when quilting lots of layers... or sewing flat felled jeans seams.

Since your 66 is from 1957, it has side clamping feet too. The same attachments will fit both machines. Your later 66 prob. has the stitch length with numbers and reverse.

As far as shaking the cabinet when it sews... is it actually mounted in a cabinet? Or just sitting on top of a surface? I would mount it in a cabinet. The machines are heavy and the rocker motion of them sewing is 'heavy' too. Just sitting on top of a surface, I'd bet would create the wobbles on both machines. With the heavy machines, mounting is the way to go. My big (new) Janome just sits in my Horn cabinet. But all my full sized, old girls... are all mounted and the grub screws are screwed into the hinges on the cabinet.... when I use them. If you don't have a singer cabinet, find a local sew friend who does. Swap out her/his singer head for yours, and try it out. Does it still wobble?