Hi HazelJane,
I have both the 201 and 15-90 in treadle cabinets.
I'm a quilter and have found that the 201 is wonderful for piecing quilt blocks and straight line quilting, not so much for free motion quilting. FMQ with the 201 isn't a fun experience, I get lots of skipped stitches and its just a bad idea. The 201 is the smoothest, quietest machine I've ever sewn with.(I have a herd of machines)
The 15-90 is great for piecing,free motion quilting and straight line quilting. I've done FMQ for years with a modern machine(Bernina) and its got a Singer 15 bobbin configuration basically.
Sorry this post has been so wordy. :-D