I had this conversation with someone a while back and we came to this conclusion:
Rosberg is 10/10 all day, every day. He's always there. If you give him a inch you lose.
Hamilton is better. He can turn it up to 11, but also bumble it and drop to 9. He is better than Rosberg, but not all of the time.
I'm pretty sure if you made a theoretical fastest lap by picking the fastest sectors for each driver Lewis' would be faster than Nicos at every weekend this year. But most of Rosbergs would have been part of the same lap.