The drop in bobbin on the 66 was the newest feature at one point, though by the time model 66 were given a backtack it certainly wasn't anymore. The 15 on the other hand is an every older model, but for some reason its' popularity has kept in high regards up to our days. In my experience, all these features has a lot to do with what we get accustomed to. I have had my 201 for well over a year know, and I haven't used my zigzagger nor my computerized machine much at all since. It has to do with the type of clothes make and projects I do. It sort of became a sport to manage on the 201. It's certainly is a smooth machine, with very nice buttonholer attachments. Either of the 66 and 15 should be equally lovely to work on in my mind. The 15 has a very good reputation when it comes to darning and quilting.