Hard-sided luggage, also known as hard-shell luggage has come a long way. It used to be you went with hard for maximum protection and soft for minimum weight. Today, hard-sided luggage is made with such high-tech plastics as ABS (acrylonitrile butadiene styrene), polycarbonate, and others that are lightweight and durable. ABS is the lightest, but polycarbonate is more durable. The most durable, but also the heaviest, is aluminum.
An interesting feature of hard-sided luggage is the 50/50 split opening, in which you pack on both sides equally, stabilize the contents with an x-strap or middle divider, and close it up like a clam shell. Of course you need double the surface space to open a 50/50 split. Most hard-sides are built this way, but there are some on the market that have a top-lid opening.
Pros: Hard-sides are best for protecting the contents of your luggage from breakage. They also provide better security because they have integrated locks and can't quickly be ripped open with a blade. That said, most composite plastic hard-sides close with a zipper, which can be vulnerable. Aluminum luggage, on the other hand, most often has metal draw-bolt latches instead of zippers. Hard-sided luggage stacks easily, making it ideal for cruise ships, which stack the baggage in the belly of the boat before departure.
If you tend to overpack, a hard-sided piece will rein you in. The case is not very forgiving, so there's no chance of over-stuffing it. For carry-on, this guarantees that the piece will fit easily in your airline's luggage-sizer.
Cons: They scuff and scratch easily; they are inflexible, so you can never squeeze just one more pair of shoes inside; and you need a fixed storage space for it. Also, few hard-sided suitcases have outside pockets.