I would personally use an internal if you could just for the reason that everything uses USB ports now, so the less you can use with other devices the better.
Speed, well USB 2.0 goes at 480 mbps and a normal PCI goes at 200 mbps. I don't know if they have PCI express for wi-fi cards.
Even if they did, lets pretend you have a PCI express 16x wi-fi card which can transmit data at around 6 gbps, there is NO WAY your internet is going to go that fast, especially wi-fi. You are probably capped at around 54mpbs or 100 if you have N.
So long story short, it doesn't matter because wi-fi speeds are not up to par with the bus speed that USB 2.0 or PCI can handle. So go with what is cheaper and more convinient for you.