To sum-up the previous posts:

USB: Mathematically slower than PCI-E, but you do not feel the difference as a regular user. A good choise, because you can usually position the antenna to a higher level / visible place, increasing the signal's strenght.

PCI-E: With cheaper budget, these will be worse than USBs just because they are usually located behind your computer, gaining a low signal. If you, how ever, choose a good card that costs 100-200 euros, you will for sure gain a more stable and enjoyable WLAN-connection.