thing I like about the usb adaptors is there easy to move to one computer to another [portable] just pull it out of one and stick it in the next as needed quick pcie card is pretty much stranded to single use one machine [opinion]
another thing is look over there support to see how well there updated some sold new today may not of had a driver update from as far back as 2000
then knowing what chipset is used in then like me I want the one that covers all windows and native to Linux as well [plug and play] not spend a hr hacking away in the Linux console to make a driver work ?
I been using this onr for years now
http://www.tp-link.com/lk/products/details/cat-11_TL-WN821N.html
its a rev 2 with a Atheros chipset in it not a realtek as the newer releases are now and is plug and play under Linux
not to say its the best but several years of service aint bad and still on it today and right now .
if buying today I think I would have to get one that covers 5ghz and full native OS support and current drivers