My hard side cracked around where the handle attaches to the case. It isn’t bad yet -but I think I will get soft next time
I have a large hard shell for when I check bags, but otherwise I have a small hard and a couple of soft that I use for carry-on or car travel, including an LL Bean wheeled duffle which is one of my faves because it’s the best of both worlds - it collapses for fit but has wheels for easy transport.
When I saw “hard shell vs. soft shell” I thought you were talking about lobsters! 
Check Target also. I looked within the past couple of years for both a wheeled carryon and check in bag. I found an excellent soft sided piece that was very light weight- American Tourister for checking in. Check around for sales and try to get the lightest check in bag- and larger than 24" if you can. Be sure that wheels are “spinners” that can easily rotate the bag. I don’t see a need for matched luggage.
I like soft sided because you can open up a piece and have the whole thing open as one side while hard tends to force you to choose halves. There are two types of carryons. I still use my 30plus year old soft bag with three compartments that can be shoved under the seat and offers nice organization- can put in a few days of clothing plus toiletries et al while leaving the big bag untouched in the car trunk or checked. Then I have the newer rolling bag with spinner wheels for short trips.
Lightweight is the key. Also spinner wheels.
Now why would you post something like that in response?? Not useful at all.
Another fan of soft luggage - we got a clearance 3 pc set from BJ’s last winter that included a large and small suitcase (big enough for a couple days clothes but small enough to still fit in the overhead storage), plus a smaller carryon that we don’t use. The set was regularly $89 but we got them for $39 - so I got a set for each of my pups.
Besides the spinner wheels, something else to look for - two quality zippers, and a pull out extended handle to make it easier to pull through airports - some of these are very cheaply made.
There are restrictions on bag size and especially weight - I like to bring my own firm pillow when I travel so I like the big size. We had found an even bigger LL Bean suitcase at their outlet store, but DH pointed out we’d be charge more for the size, and if not, I would probably overstuff it beyond the weight limits.
One benefit of the hard luggage we had years ago is that it was easier to apply bumper stickers to it - from where we travelled. They don’t stick well to the soft luggage. So I stopped buying the souvenier stickers.
Re: spinners. It depends on where you are traveling! Spinners would have been useless in the foreign countries where baby kid lived. They were no match to the cracked asphalt and dirt paths on which she had to drag her luggage.
The spinners can be used as a cane as you rush through the airport after twisting an ankle. Don’t ask me how I know this.
a little late to the party so I didn’t read all the replies, but before buying anything I suggest asking your DIL about their travel habits.
Some people travel with only carry-on luggage. Nothing gets lost at the airport, it is easier to take on public transit and thru the stations such as when changing trains. Books such as those by Rick Steves recommend this pack-light approach, one of his guides has a talk at https://www.ricksteves.com/watch-read-listen/video/travel-talks/packing-light-right-with-sarah-murdoch or info at http://adventureswithsarah.net/ Others like to carry more with them, willing to trade off convenience for bringing more.