From my past experiences, an external TV-tuner is a much better choice. The current one I have (which I highly recommend) is ViewSonic's VB50, which I purchased from www.newegg.com
The reason why external ones, I find, are much more convenient, is that you can use them when your PC is turned off. As well as most of the external ones come with a remote control, allowing for much more functionality. Currently I have a Gamecube, NES, computer/monitor, and my surround system plugged into it, and everything works like a dream. Also, you arn't always having to fiddle with cables behind your PC tower, and it's much more transportable; not being an actual part of your computer. And if you were to sell your computer, you could easily keep it for your next one, and so on.