My wife uses and likes the Exo 3500. I have the Exo 5500 so it works great if she isn't going with me, I can put her 3500 on my frame if I want a smaller bag.
If those MRs are a little too steep you may want to just make an REI run and try on some packs and see what fits best. Like Becca said, get a little bit bigger than you think you need, it can only help..