In practice, I haven't found the 201-3 or the 15-90 to be unable to do anything that the 201-2 or 15-91 would do, including old hard naugahyde. I've had all 4 here at one point or another. I still have the 201-2 and the 15-90.
In fairness, I haven't tried some of the ebay tests like piercing pop cans and such, so maybe my 15-90 or the 201-3s would object, but I wouldn't subject the 201-2 or the 15-91 to that either.
I don't like that if the fiber gear in the 15-91 or 201-2 strips, that the gear is close to impossible to replace, essentially turning that motor to garbage. I don't subject those machines to too much because of that.
For the purposes of most of the people on this forum (quilters), the 15-90 and the 201-3 (201-K is the same thing - belted motor, light on the back) are more than robust enough for what we're going to throw at them. My first quilt was FMQ'd on the 15-90. Not even a burp out of it.