If possible, a backpack is a good thing to try on and test for comfort and where the straps fit on you. (In our Edmonds, WA store, display bags are already packed with 20 pounds of junk for you to try.) A waist belt is important to me to distribute some of the weight. Even in an airline carry-on sized bag, it's easy to pack more than 20 pounds (especially of shoes, toiletries, books, liquid souvenirs), so be vigilant. CDG has free luggage carts, so grab one as soon as you see one, if your bag does not have its own wheels. You can end up walking much farther than you expected at CDG.
I have used a backpack as my main travel bag for years and appreciate it's advantages when navigating stairs and rough roads, but also end up carrying it more than necessary, especially when I stand in a line that I think will be short and it turns out to be long. Wheeled bags are used by the majority of Rick's customers and by the majority of European travelers, too. Carrying a backpack through CDG, you will find yourself in a minority, but at least it won't be the group with the huge suitcases.