I had a WinTV-HD at one point, and replaced it with an accessDTV. Overall, the accessDTV and its software seem to be more stable than the WinTV-HD. Channel-changing seems faster, too.
The ATSC tuner in the WinTV-HD is supposedly capable of decoding QAM, but I don't think the Hauppauge software supports that yet. I believe the accessDTV can decode QAM (assuming it's unencrypted), but I haven't tried that. Maybe Cliff W can comment on this capability.
The big difference between the two is recording capabilities. Right now, the WinTV-HD has no recording at all. It remains to be seen what they will offer in future updates, but my guess is at best it will be on the level of what the HiPix offers.
The accessDTV, on the other hand, offers recording out of the box, and can also pause live TV. It has an optional program guide that can be used to schedule recordings, but you can also schedule manual recordings. It's still missing some functionality that the set-top PVR devices include, but it's the only HDTV time-shifting device you can buy. If you want to add HDTV to your HTPC and you want recording capability, the accessDTV is the way to go.