Ive read through a ton of topics (thanks ken!). I am traveling with my sister for 4 weeks all over Europe. We will not be hiking that much with the backpacks on, however I don't really want to do the convertible suitcase because they seem harder to carry on and I already have a few suitcases so purchasing another one to fit European carry on standards seems wasteful. She is planning on taking her school backpack so I expect I'll have to carry some of her stuff in addition to mine. I have found I like the recommended Osprey Farpoint http://www.ospreypacks.com/us/en/product/farpoint-70-FARPNT70_663.html.
Is this a good choice for my needs? What size should I get? It seems some people think that the 70 is far too large and others tell me it wont be big enough. I expect that as my trip progresses, I may have to check my bag due to weight from purchases.