Okay, this is my opinion. A PCI card is better for the following reasons:
1.When I had a USB wireless device, every time I would switch things around, I would have to reinstall the drivers for it. It hated being unplugged and the thing was just temperamental. This actually has been a problem with most of the USB wireless devices.
2.The PCI device has no chance of you unplugging it by accident unless you are going into the computer.
3.The cards are usually well supported.
4.I have had overall better luck with the PCI wireless cards.
For Windows 7 64-bit, I would go with one from Belkin or Netgear. Both of them have had great support under the 64-bit system and I have had great success with both of those style cards. Good luck with your choice and remember, if this helped, do not be afraid to thank me lol.