I'm born and raised in far northern California and lived in buffalo. Therefore I can tell you there might not be more polar opposite fan bases than bills/niners. As a Bills fan I find them probably the most annoying fan base (even my 2 best friends are niner fans).
Niner fans are extremely fair weather fans,(wierdly the same with sf giants fans) yes there are true fans that support them thick or thin but the mass majority of them disappear when things get rough and show up louder than anyone else when things are great. Let's just say there were almost zero niner flags flying in California during the jeff garcia days. Then the harbaugh sb run happened and wouldn't you know it everything turned red and gold. Wether it was Trent Edward's or josh allen, buffalo flew flags.
I currently work with many niner fans, and I love messing with them, I tell them to name 5 niner qbs without saying young/Montana, and about 1 out of 10 niner fans could do it.