I would probably lean toward the 80L, mostly because the side bags can be compressed to hold less. If you need to store more stuff, or stash away layers, you have room for them. Now, if you're an ultralight fanatic and all your gear is ultralight, the 40 may be the way to go.
My thought would be that the 40 would still require that I use a tail bag for tools, tubes, etc. The 80 would allow me to store it all in one bag, but still be segmented, due to the design. Just my thoughts, good luck with your decision!