Computer bag brands come from two worlds: specialists in computer protection and luggage manufacturers. The former offer better protection, while the latter are better for travel.
Two families of brands share the computer transport market. Protection specialists design first around the machine: padding, reinforced corners, and compatible sizes indicated very precisely. Luggage brands start from the journey and add a padded compartment.
A laptop bag advertised for 15.6 inches accommodates a machine whose screen diagonal does not exceed this value. The size in inches measures the screen diagonal, never the bag. Serious brands also publish the maximum machine dimensions in centimetres: this chart is what you need to read, not the mention on the front.
Three questions are enough. Is the compartment padded on all six sides or just on two? Is there a raised base that protects the machine when you put the bag down harshly? Does the brand offer laptop bags in various sizes, or just a one-size-fits-all?
The warranty matters more here than elsewhere, because a bag is opened and closed several times a day. Check if it covers the zipper and shoulder strap, which tend to fail before the fabric. A brand that sells a replacement shoulder strap saves the hassle of having to buy a whole new bag.