Size matters, inside and out. The inside should hold lots while the outside stays as small as possible. Avoid protrusions like exterior pouches. They can put you over the size limit and throw your bag off-balance. And be aware some handles take up a lot of room inside the case so check before you buy.
Next, consider the wheels. “Spinner” bags have four wheels and maneuver easily in crowded airports and plane aisles. They can be pushed along, taking the stress off your shoulder and elbow. Spinner wheels are vulnerable to breaking off, so make sure they’re attached with screws instead of rivets. If you prefer dual wheels, they should be set far apart. Too close together and you can lose control.
And before you buy, open the handle all the way and give it a shake. It should be firmly attached with no wiggling or rattling as you pull it along. And then there’s the zipper. It’s all that stands between your stuff and the outside world. A cheap zipper can render your bag useless.
Many bags come with a lifetime warranty. That sounds great but read for exclusions for airline damage and wear and tear.