About six months before I built the MythTV system, I started with a Hughes HTL-HD set-top box. Back then, I could tell that the tuner in the HTL-HD was significantly better than the pcHDTV card. When I was struggling to get the pcHDTV card to lock up on a signal, the HTL-HD would give significantly better picture quality. Now, the situation is reversed. In almost all cases, the Air2PC card tuner performs significantly better than the HTL-HD tuner. It’s common for the Myth to lock up on a channel and receive a perfect picture while the HTL-HD struggles.
MythTV has a function in LiveTV playback that is supposed to report signal strength and quality (hit F7), but the function doesn’t work with the Air2PC driver. As a result, I can’t really conclude in a quantitative way how much “better” the Air2PC card is. I haven’t tested the pcHDTV HD-5500, but I would assume it is comparable to the Air2PC card. I believe they both use the same late-model LG tuner, though it’s hard to tell from the data sheets.