Just for the sake of being complete: A duffle bag will not prevent your bag from being gate checked. Chances are considerably lower, but exist never the less.
On my early FR flights, I used to carry an Eastpak duffle without the floor + wheels. It can be folded together for light luggage and go on board, but is tough enough to be filled to the brim and travel around the globe. It can be carried as bag pack. Big disadvantage: It's not waterproof. While the zip is the weak link on plastic/aluminum suitcases, water gets in literally anywhere on the Eastpak duffle.