First off all hd stuff is ripped from Blu-ray. So the ultimate would be to dl the actual blu-ray. For that you'd need a blu ray burner and blank disks which would cost you an arm and a leg.
here is a cutandpaste re the formats
There are two main flavors of high definition TV, 1080i (the i is for interlaced) and 720p (p for progressive). 1080i offers the most pixels, with a matrix of 1920x1080 pixels, while 720p has fewer pixels at 1280x720 pixels. However, the difference is made up with the frame rate, which is only 30 frames per second with 1080i, but is double that with 720p, at 60 frames per second. The total pixels displayed per second is actually very similar, with 720p offering 55 million pixels per second, while 1080 is slightly higher at 62 million pixels per second.
What does all that mean? It all depends on the type of TV you watch. 720p is better at showing pictures with plenty of motion, since the higher frame rate helps smooth any quick motion on the screen - this is better for sports or action movies. 1080i offers more detail, which is for movies with lots of images or panoramas.
The best of all is the 1080p option. It offers the best of both world, 60 frames per second at 1920 x 1080 pixels. The toal bandwidth is 124 million pixels per second, double that of 1080i. It can display any HDTV signal without any downconverting. 720p signals are upconverted, while 1080i signals only require some gently "de-interlacing" to work properly. 1080p is the perferred option is possible - its backwards compatible with all old formats, and is ready for upcoming high definition discs.
None of this has anything to do with audio, i know but if you're burning actual blu rays then it should have the best audio available.
cheers