What to consider when we buy a backpack?
First, you need to consider what kind of backpack you want, for working, travel, hiking, sports or school, that's the usage of the backpack. As there are business backpack, laptop backpack, travel backpack, hiking backpack, school backpack, outdoor backpack and sport backpack.
Second, what material you like. There are a variety of materials such as 600D, 1680D, Oxford cloth, 420D, 900D, polyester, nylon ect.
Third, the style you want, simplicity, casual, elegant ect., depending on your preference.
Forth, that's the size, there are bigger and smaller size, normally a backpack with 18 or 19 inch is more suitable for working and laptop, and school, even for travelling, bigger size may be better and suitable.