The 115 is lighter duty, or at least it seems it. The whole hook mechanism is less robust looking.
They also share a bobbin case and bobbins with a 20U and a 306 I tried hard to bond with the one I have here, I made a case for the lexan inserts for my custom desk, and I swore at the machine left and right. It's fully adjusted and working, but for some reason, it balked at everything I tried to sew. I suspect the motor, but I thought the motor was fine on another machine. *shrugs*
I'll get DH to pull the 115 down from its shelf (above my head) and post a pic of both if someone else doesn't get to it first.
According to Alex Askroff,
http://www.sewalot.com/singer_through_the_ages.htm
The 15 was in production in 1879. The 115 I have here is a 1917. I think they ran concurrently, but I don't have a date for the beginning of production for the 115.