Hmmm. My friend says she is thinking of getting a vintage machine and wants to know which I would pick if I could only have ONE! Who would want to only have one? Three seems more reasonable.
I have been wishing for a 411 because it has good zigzag and is treadleable. But really, many vintage Zigzaggers can be treadled from what I've read. I highly recommend timquilts.com and his YouTube videos for free motion quilting on various treadle machines. He says a lot in his commentaries about the issues with different machines and why he is using the darning foot for each project he is demostrated. He seems to be mostly FMQ on his singer 66 treadles lately. I bought my White Family Rotary treadle ($40) for the big harp space (9+ inches) and I love the smooth balance of how it sews, but can't get top clamping darning foot. Tim has demonstrated using his WFR for free motion with several different work arounds, but I haven't tried them yet.
The Singer 15 is spoken well of, but the 66 has a larger harp size and there are many different darning/free motion feet that will fit either machine. I had a Standard Rotary treadle that was beautiful, but smaller ($55). Same issue with feet as my White. I gave her to my sister who was fired up by my treadle talk. I am still watching for a singer treadle base to put one of my 66s in. It is too much of a pain to try to jury rig a way to drop a singer into the bigger cut out on a white. I didn't check CL last weekend and missed a nice singer treadle for $25. There are so many treadle machines in SoCal craigslist, I can afford to wait for the cheaper ones;-)