It kind of depends on your gear. If you're camping in cold weather and bring a big down sleeping bag and a big down jacket than a bigger pack might make sense for you.

I backpack here mostly in cool to cold temps and I can squeeze, and I do mean squeeze, 3 nights into an Osprey 44. Since food is the only thing I'd need to stay longer, I'd think a 70 would work good for a week long trip for most anyone in most instances. If you'll be doing longer trips than maybe the 85 is the right choice for you.

The Osprey pack will cinch up pretty nice when it's not jammed full, so if the weight difference is minimal and you'll use it on longer trips, the 85 may be the ticket.