The pros of hard is, in my experience, it lasts longer. It is also good for protecting fragile and delicate items. Whenever I traveled internationally, I’d always take a hard shell to protect the inevitable bounty of arts and crafts that I would purchase on the trip and nothing was ever damaged. I find it easier to pack, and items are more easily accessible.
The cons are that hard is heavier (but a 24" wouldn’t be bad), and yes, more difficult to fit into a spot because it’s rigid. I don’t feel that they are harder to maneuver though, since everything is on wheels these days.
I got a set of Samsonite hard luggage for my college graduation over 30 years ago. I didn’t leave my home state until I was 16 and when I got my first professional job after graduation, I expanded my list of states visited to three as it was two states away. I started dating one of my coworkers and when we planned to go on a short trip, he made a sarcastic remark about my nice luggage set, saying “Why do you have such nice luggage? You’ve never been anywhere!”
Well, I married that knucklehead, and we spent the next 30 years travelling all over the world - and that luggage lasted about 20 of those years.
We got another set of the same brand (I think at Ross), but sadly, it only lasted for about 7 years - I guess the quality was not the same as it was 30 years ago.
The pros of soft are what you mentioned. Plus I think you can squish a lot more stuff into soft luggage like a wheeled duffel. The wheeled versions are very maneuverable, but you can’t compress them as much as a totally soft shelled piece as they have a rigid bottom.
A few years ago, I bought our whole family of four a set of LLBean rolling duffels (each in a different color) with a matching small duffel for the carry-on. But I don’t like the lack of organization with duffels (one big compartment) as opposed to the hard luggage I’ve had. Even though you can get more stuff in - I have to use big ziploc bags (the storage kind in 2 gallon and bigger) for each type of item - like one for socks/underwear, one for shirts, etc. just to get our stuff organized somehow. Otherwise, it all ends up in a big jumbled mess after a couple of days.
My husband has traveled the most with his set and his big duffel showed signs of wear after a few plane trips - where luggage seems to get more abuse. My daughters and I have traveled mostly by car and train, and ours still look almost brand new. I do love the small duffel - it has a shoulder strap and I use it like a carry on with all my personal items and it works very well for that.
If I was going to get my daughters a set of the size you are thinking of, I think I’d actually mix a hard shell 24" inch piece with a small soft duffel carry-on. The LLBean ones come in many colors and you could probably get one to match a hard shell piece made by another company.
https://www.llbean.com/llb/shop/63911?page=adventure-duffle-small