There really should be no difference in performance assuming the your computer is USB2.0. However, PCI is almost always more stable and tends to cause less "lag" than USB devices. With something like this, especially if you're just using the internet, you really shouldn't notice a difference.
Eh, I guess basically, either would be fine, but personally, I'd go with PCI.
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Desktop * Athlon X2 4200 | 2G RAM | 160G HD | 7600GT
Laptop1 * C2D T5550 | 3G RAM | 120G HD | Intel x3100
Laptop2 * C2D T5250 | 2G RAM | 160G HD | Intel x3100
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