2) Potentially a bit more flexible in terms of where you locate it; PCI adapters will restrict your aerial position to somewhere round the back of your tower near the bottom whereas if you have front and back USB ports you get a few more opportunities in terms of where you place it - could help if you are a long way from your WiFi hub.
3) Arguably easier to setup for novices i.e. no need to insert anything inside the case.
4) Modern PCs tend to have stacks of USB ports and not many PCI slots
Personally I would take PCI over USB anyday