Desktops offer a wide range of configuration options even after the device is purchased and installed. For a person who needs higher capacity (multiple internal hard drives or extra memory) the desktop is the most likely candidate. Laptops are very portable but the configuration you buy is pretty much the way it will be forever. Tablets are just getting restarted and we will see where they settle but they are available with only what the manufacturer wants to install. Few, if any changes are possible.