I love my 15-91, I'm not familiar with the 301 though. The 15 has a feed dog that drops and it sews through thick leather and the finest cotton, all without changing the tension! I love this machine, I call it the tank.
I re-upholstered a chiropractor's table with 10 different sections. The leather was a full 1/4" thick! He ordered the leather from New Zealand, I didn't know cows came that big!
The tank didn't even hiccup. I used an 11 needle for leather.