I use an Osprey Aether 70L for everything (except day hikes of course).
In my experience, always buy a bigger size when in doubt. Not only does it give you more flexibility when you need to carry a lot of volume (many days resupply, for example), but it also allows you to get a better weight distribution, using the compression straps.
If you have a small backpack and really stuff it, the centre of gravity tends to be further away from your back and relatively low, which I find less comfortable to carry. However, a larger pack allows you to shift the centre of gravity further up, closer to your back and more above the hips, making it a more comfortable carry. I always see people counting grams, trying to optimise their setup using a kitchen scale, but I really don't think that is the right approach.
I'm very impressed with the Osprey customer service. My pack has had lots of abuse on thousands of miles walking, usually stuffed with lots of food and gear. I've had several repairs (replacement of straps, patching up tears, replaced buckles an draw cord, etc), all free of charge! All these things had nothing to do with build quality (which is excellent), just normal wear and tear. I just want to keep this pack alive because I love it, so I have always told them that I was willing to pay for the repairs if necessary, but they have never charged me anything. Unbelievable!
Btw: other companies that are very good with their warranty is Therm-a-Rest (got my NeoAir XLite replaced free of charge when it developed tiny leaks on the welds after intensive use) and Platypus replaced 3 of my 2L bladders free of charge (which was also totally normal wear and tear by any reasonable standards).