Both machines will sew anything you can cramp under the presser foot, but especially the 15k is good at heavy stuff like denim and the occasional bit of leather and ducking. I suppose the quilting and darning feet that came with these were as frequently used as the straight stitch foot…
But again; don't use a 15k for speed sewing; it'll walz right off the table! I own two 15 k's; one built in the very late 30's that sews really nice and was offered to me by a tailor who told me it cost him three months' wages. Another, really late one that has the retractable feed dogs (for embroidery and stuff I never use), that has a little more "clicketyclack" than the older one. Someone offered it to me; must have thought it was time to replace the old one. Then, last year, the retired tailor asked me to pick up his little 99 Singer; he was moving out of his house and wanted to get rid of it. So I gave him 50 Euro as a gesture and took it home. It is a cute "3/4" size machine that I would actually recommend anyone who sews fine and light fabrics; the needle opening in the stitch plate is the smallest I have seen to date. My 201 is from the early 50s, the latest batch of the cast iron black beauties and a dream to sew on, my absolute favourite. Like you, I use computerised machines (Bother) to do buttonholes, bar-tacks and things but I have never sewn a straight seam with them; the buttons and needle-up and controls drive me batshit-crazy...