PCI performs better for me but may be a little more fuss to set up. Any extra gain from wireless N depends on both router and adapter being N -- and I believe you're still safest buying both from the same manufacturer.
I've tested both using speedtest.net and the server for my internet and it seems that sometimes the usb gets a faster speed, but most of the time the pci has...they were both so close though that it is not evident either one is better yet...does anyone have any experience which has indicated pci being better? i want to know if i should keep this usb adapter or not. i just got it yesterday so i still have time to return it if pci really is better..