I would ignore what REI says you need for a certain length trip. Or any other estimate for that matter.
I want most of my gear to fit INSIDE my backpack. I carry water bottles and my crocks (use for stream crossings) on the outside, and fishing pole when I take it. I do not carry sleeping pad or tent strapped on the outside. This is because I do mostly off-trail travel and do not want to damage or loose gear. Additionally, strapped on gear can make your pack off-balance, particularly if it swings around. IF you think you will have to strap on gear on the outside of the smaller pack, I would go with the larger, given the little difference in weight or price.
That said, the pack I mostly use is about 60-65 liters. That goes for a 13 day trip, including a bear canister on the inside. Because most the areas I hike require a bear canister I cannot use the 40-liter packs. I have a Bearikade Weekender and have to bring it even for a 2-3 day trip, so the volume requirement is not much different than for a 9 day trip. If I have to carry other peoples gear (taking the kids, etc) I go to an external frame Kelty pack that I have stripped down and modified to weigh about 3.5 pounds.
You should buy the pack, take it home, and inside the house spend some time packing it, trying many different methods and gear locations. Packs with the same volume can pack very differently depending on how the volume is distributed - all one compartment, shape of compartment (more square or more rectangular) several, how many pockets, side of pockets relative to the size of gear you wish to put inside the pockets. Pockets are frustrating to me- never the right size! If it does not work, return the pack and get another.
A larger volume pack is not necessarily bad as long as you are disciplined at not filling it up every time just because it fits!