Sally,
Keep both machines! The featherweight is a great little machine and sews well too, its small & easy to store. I own
three featherweights and cycle through all of them, a favorite for quilt classes and quilting group. I love the sound of the FW, plus they are just darned cute!
The 15-91 will just about sew through rocks, I have the 15-90 and have hemmed blue jeans with her and it was so easy to sew. IMHO all the Singer machines of this vintage are great machines and sew well, easy to learn and to maintain; they just need a bit of oil here and there.
I treadle my 15-90, great machine. Its my favorite for machine quilting.
Sharon W.