It's a personal thing, we all agree. I don't mind filters, didn't use any M lenses on film cameras previously, and didn't want anything fast besides one lens in the 50-90 range. Considering those preferences, the real difference between M8 and M9 for me was iso. I didn't have the funds to buy an M9 when I got my M8, and I didn't want to wait around to build up the money to get an M9. The M8 was 80-90% of the M9 to me.
Now, however, I want a small 35 that's f2 or faster. That you can't have on the M8. Anyone who considers that a deal breaker for their camera would say the M8 certainly isn't 80-90% of the M9 - on an M9 you can buy a summicron, summilux, or nokton, all small and f2 or faster.
It all depends on how you use the camera.