What I would call a backpack, I'd never take. They are longer than the carry on maximum and have to be checked. My goal is to always carry on my bag. I find that I rarely carry my bag more than a quarter mile and a real backpack isn't necessary.
Carry on only is so convenient when you're traveling. You can hop on a bus, metro or tram easily while people who are loaded with bags need to hire a cab to get anywhere. I also love being straight out of the airport and on my way when I land in Europe. I don't want to wait 30 minutes or more for my bags to appear - or worse, spend more time if the bags fail to appear.
If you want to carry on, you have two choices - a convertible carry on (a Rick Steves type bag with shoulder straps) or a roll on.
I prefer a convertible and I like this Campmor (https://www.campmor.com/c/campmor-essential-carry-on) because it's the lightest option (and really inexpensive). I've had other brands that cost triple. This one seems every bit as sturdy but weighs about half of what others weigh.
Weight is important because many airlines are now limiting your carry-on by weight as much as size. Lufthansa, SAS and many of the regional European carriers limit your bag to 18 lbs. The less your bag weighs the more you can allow for your stuff.
Weight is my main problem with roll on bags. The lightest roll ons weigh more than a convertible. More often than not, they weigh doulble the heavier convertibles (four times my Campmor). If you go to four wheel versions, they're even heavier.
That said, most of my family take roll ons and are happy with their choice. They manage to keep theirs under the weight limit. Roll ons are great in airports (while I'm lugging my bag). My bag beats theirs on stairs and any uneven ground or cobblestones.