The WinTV-HD and the Hipix are pretty much equivalent in terms of the hardware. They do use different demods, and the WinTV-HD can use video overlay to show an image on your computer monitor while viewing HD on another. The HiPix uses a VIP connector that is not well supported by graphics card manufactures.
Both can record, but at this time the HiPix software is better. It has more features and seems to be heading in the direction of a HiDef PVR. The WinTV-HD is also moving in that direction but at a slower pace software wise. I expect that at some point third party apps will appear that replace the manufacture supplied software and add features that are more useful to an actual home theater user. That will take a while though.
One important point is that you can actually purchase the WinTV-HD, while the HiPix has seen some manufacturing difficulties and is currently on back order. I'm not sure when the back orders will be filled, but it should be sometime in the next month.