I have just bought a hardside spinner bag from Target, used it on 2 trips so far, and I love it. This is heresy on this board, but that's ok by me! I used to say spinners aren't good for cobblestones. That's true, to an extent, but I've found that even in Europe 80% of my walking with the bag is done on normal sidewalks, and I can manage the other 20%. The reason I prefer it is because I can fit more, with more organization, in it. The hard side give it structure that I can fill. I also have the softer Rick Steves rolling bag - it's very nice, I just worry about overfilling it, I'm not all that neat so bunching up dirty laundry upon checkout leads to a lumpy mess. There's no possible way to overfill a hard bag! This is probably specific to me and my quirks, though.
Also, the 2 compartments in a hard bag make for good separation. Clothes in one side, things like toiletries, electronics, shoes in the other. Mine weighs 7 lbs, the Rick Steves roller is 6lbs. That can make a difference, but I've managed to pack it under 8kg total. 10kg is no problem at all, to get to 8kg I do have to strategize a bit.
I do have an ebags motherlode jr backpack that I like and use when I am struggling with weight limits. However, I just don't find 15+ pounds on my back all that comfortable. I'm in decent shape, but I'm also 5'3 with tiny shoulders and a short torso and every backpack I've tried fits strangely. The Rick Steves backpack is impossible for me, but my husband likes it. The ebags backpack is the most manageable I've found for my size. If your party includes any women on the smaller side (or kids/teens of either gender), they will want to try on any possible backpacks FILLED before committing to using them. Not just try for a minute in the living room - fill the bag and go for a walk.
Remember you don't need to get 5 of the same bag. One person might prefer a backpack, another a rolling bag. As long as all can manage their bag (or you have a solid plan for how to carry the bag of someone who can't) it all works!