my 15-91 out sews my 201k under heavy loads. The 201k is a little smoother. They are both potted motored machines (no belt, gear driven). With the 15 I can use t90 thread and a number 18 needle no troubles. 201k is a little picky with t90, 69 is fine. Over all I think the 15-91 is a better machine for working at these machines absolute limits. Also since they are home machines they see very little use compared to the commercial machines, both old commercial machines I own are realy beat up and show it inside and out. So I'd go with something newer if its going to be commercial. I have picked up a lot of 15-91s over the years and I would say don't pay more than $75-100 for them locally.
I only use these for canvas work. I feel they are not really leather machines.