I have both machines, both potted motors. To be honest, I prefer my 15-91 to the 201-2. Couldn't tell you why, other than I have had no tension issues with the 15, and I had to fight that 201 every stitch of the way to a good seam. Most likely NOT the machine's fault, someone had messed with the tension before I got it. When I got the machine, the seams puckered terribly. It took a lot of adjusting to finally get it right, along with several repeated swear words and even a couple of tears of frustration. I now associate that machine with those things!
Even so. Both machines are quiet, hard working machines. Both have a beautiful stitch now, and both are pretty machines. Seems to me, other than a small difference in size, the main difference is that the 15 has a vertical bobbin while the 201 has a drop in bobbin. The vertical bobbin has worked better for me for FMQ...I still get eyelashes when I try to FMQ on the 201, and with other machines to use, I let her have her way and only use her to piece with.