#1 What is the difference between the standard brush roll and the premium?
The Premium brush roll is apparently certified by the Carpet and Rug Institute to meet criteria for leaving carpets in good shape. The NV370 is certified, the NV356 is not on the list. It might be that the extra money is due to the certification. Shark's page doesn't even list the 370. Search "Carpet and Rug Institute" for details.