TV Tuners For Computer Recommends

I am going to get a TV tuner for my computer so I can use it as a DVR. I just got an external disk. I was looking at the various TV tuners. I was wondering what is better an external (USB) tuner or a internal tuner card.

I don’t have a laptop, but may get one in the next year, so I was thinking, it might be handy to be able to move the tuner, if I wanted to use the laptop as a TV as a temp device.

If you have one, what are your recommends? Do you find one type has an advantage in quality or is it just convenience?