Very true, however there can be the odd exception to this, for example playing a hardwarte taxing game online. The game may already be putting a large strain on the CPU meanign it can provide less juice to the USB device decreasing internet performance. As such in day to day terms there is no difference in actual connection speeds, however on soem occasions when playing games there can be a slight boost by using NIC over USB. Either way i would always give a PCI NIC the nod ahead of USB
You normally get better system performance, not Internet performance from using a NIC.