The nice thing about hard-side rollers is that they are like a big compression cube. My Monos roller is only about 5 liters bigger than my Minaal Carry-on, but I feel like I can stuff about 20 extra liters of stuff in it since the hard sides help to hide just how compressed the stuff is inside. That said, I always feel like I can't do one-bag travel with it because I fear the forced-check at the gate. I always need to bring a topper backpack with electronics and medicines in case they force me to check then lose it (s**t happens!)
How dare you recommend wheels?? :). I used to do backpack only but converted to wheels when I tore my acl and was worried (probably for no reason) to carry 40L on my back. Now I travel with a carry on 2 wheels luggage (a Briggs one) which you can roll mostly anywhere and strap a 25Lish duffel (the pakt duffel but will try the black ember dex 30 soon) pilot style so the luggage is easy to roll (and I can carry it duffel when needed for short distances). Roller in the overhead compartment and duffel below seat. I feel that I have maximized my carry on space (i always have room to spare) and transit is smoother IMO.
