I have had my eye on this coin for quite a while. The price was reduced recently had to get it. Sorry for the scratched slab.
I'll go with MS64, I'm not seeing the presence of much luster, which makes me think it was held back from a Gem grade.
The coin seems rather lackluster but I don't know if it is your photos or not. It is pretty clean but the strike is rather mushy for a date that does have some nicely detailed coins. My initial reaction was MS63 and I'm sticking with it.
I should add, I have a number of Buffalo's with that streaky toning. I kind of like the looks of those.
Funny that 2 different 36S Buffernicks get posted on the same day. I'm curious to see the comparison. As lehigh mentioned, I think the luster and strike are weaker on this one than the other one. Hence, I will wager a 64 (but Buffalo's are one of the hardest series for me to grade).