Have you considered an hybrid bag? I was in a situation similar to yours and got myself this bag: http://www.suitcases.me.uk/vaude-sap...ry-on-trolley/

Most of the time I use it as a normal carry-on and when I need to walk outside in the snow or over rough/uneven ground I use it as a backpack. A nice bonus is that the wheels on this bag can be replaced very easily at home (they are essentially roller-blade wheels).