As Frank notes, USB 1.1 is much slower than a PCI slot (actually, the limiting factor will not be the PCI slot, but the inherent speed of the WiFi equipment - 54Mb/s for standard 802.11g, faster for the proprietary speed burst and Pre-N brands; whereas USB 1.1 maxes out at 12Mb/s). However, if you are not going to be transferring large files between multiple systems on your internal network, then the speed of USB 1.1 may be fast enough for all but the very fastest tiers of cable broadband, so ordinary web surfing will be unaffected.
The upside of a USB WiFi device is greater flexibility in the placement of the antenna. If your computer placement conveniently works out for reasonable signal path between the router and the antenna sticking out the back of the PCI card, then you're fine. But if there are signal path issues, you'll need an external antenna with a pigtail extension wire so you can put it where it gets a good signal. The USB WiFi is already on a 6' cord so you can put it where is works best.