Honestly man I would go with a nice piece of carry on rolling luggage. I did the whole backpacking thing in 2010 all throughout Europe, and there is no need for a backpack - all the streets are paved.
I found that shlepping this 40lb backpack was a pain in the ass, made me look like a ridiculous turtle tourist, and ensured that all of my clothes were wrinkled at all times. It's much easier to casually meet people/ girls when walking with a rolling bag then a huge backpack.
Think this guy:
Vs. This guy
--> Both are carrying about the same amount of clothes.
Especially when you consider the size and weight limits imposed on carry on bags in Europe, rolling luggage really kills it.
I've now been back to Europe twice since then, and all I roll with is rolling luggage. Properly folded you can carry 3 pairs of pants, 4 button down shirts, 7 pairs of t-shirt, socks, underwear, plus 2 pairs of shoes and a toiletries kit. All without having to physically carry it.
I've been using the earlier (non expanding) version of this bag for 7 years now - wish I had brought it with me in 2010. It's very rugged, fits perfectly overhead, and is made by Victorinox so you know it's knife friendly
http://www.amazon.com/Victorinox-Mobilizer-Expandable-Carry-On-Size/dp/B003Z62ISS
http://www.youtube.com/watch?v=IJ_WafW8Ddw