Verified answer

The PCI card MIGHT have better range, especially if it's a model with an external antenna on the back of the card.

USB wireless adapters often have the advantage of placement flexibility. Many of them come with little stands/docks attached to a 3' usb cord, so you can put the adapter on the wall, or above the desk etc. If your computer sits underneath a desk, your wireless signal might be blocked by the desk itself, which can negate the better antenna on the PCI card.

However, PCI cards don't suffer from the problem of intermittent disconnects that USB devices sometimes do (mice, network adapters, keyboards etc)

If wireless reception isn't great, it's also worth looking into powerline ethernet adapters.

to be honust i didnt think there was much difference apart from the wireless pci card has two big rod things sticking out of it and the usb is only a little thing but maybe the pci card is alot faster than a usb wireless im using wireless in a 3 storey house as my wireless rooter is on the ground floor and i still get 54.0Mbps when it's just me on it but when more people come on i get 48.0Mbps but at peak times i just come off it lol as i only get 11.0Mbps sometimes and thats rubbish but i don't mind during this time i like to scan for viruses, defrag all my hard drives and do a system clean up.