I think if I had to only have one vintage machine, and I did mostly quilting, it would be one of my 15's - though my non-potted motor one works better than either of my potted motor machines, but each machine can have it's own personality with minor differences. I seem to have better power control with the external motor, but maybe the problem is the foot control too. BTW - do not hesitate to get an updated electronic foot control for an old machine if you find that you can't get the speed control you want.
One big reason for me wanting this machine - my long arm also takes the 15 size bobbins, so I can easily wind them on my 15, like a dozen at a time. I haven't tried winding this size bobbin on a machine that takes a different size though.