to answer your other questions. The 15 has been around for a very long, long time. The 115 was made during a much shorter time frame, but consequitivly with the 15.
The 15-30 (which is the one that is similar to the 115) was made from 1895-1933, the "next generation" was made 1930-1956 this is when reverse was added.
The 115 was made 1912 to 1935. As far as my research, it was never made with reverse.
what makes the 15 great for FMQ above some of the others is the ability to drop the feed dogs. I'm not sure if the 115 has that, and I'm currently 85 miles from home, so I can't go look.
I have recently done some FMQ on a machine that takes a cover plate instead of dropping the dogs, and I had clearance issues as the hopping foot was only going up a small amount, and the cover plate took up half that. It made moving the fabric sandwich around a lot harder than when I switched to a machine I could drop the dogs on. I was surprised at the large different that made in the ease of moving the fabric. I rarely feel I need gloves to hold the fabric, but I sure felt like I wanted them with that cover plate machine!