The short answer? 720p. Now, here’s the long answer…
First we have to understand the difference between “p” and “i”. The “I” stands for Interlaced, and the “P” stands for Progressive. Video files, DVDs, and TV Broadcasts are either coded one way or the other, you don’t mix and match. If you do, then you get an inconsistent image because it was not edited correctly. Any video signal that you watch on your TV set will be 24fps (frames per second) (movies, film, etc…) or 30fps (sports, news, cartoons, sitcoms, etc…). A 1080 interlaced video signal (1080i) means that each frame contains only half the content of the picture content. Huh? Ok, let’s say you are watching NBC’s HD brodcast (1080i 30fps). Each second contains 30 frames. The odd frames contain 540 horizontal lines of the first image and the even frames contain 540 horizontal lines of the next image. Like, when a car goes by the screen, left to righ, the even frames contain where the car is going to be next, while the odd frames contain the current position of the car. The odd frames are called the “Odd Field” or “Upper/Top Field” and he even frames are called the “Even Field” or “Lower/Bottom”. Since the broadcast is 1080 lines, the Odd Field contains lines 1,3,5,7,9,11,13, etc…whereas the Even Field contains 2,4,6,8,10,12, etc…
Now…we as humans have a hardtime seeing just 1/30th of a second, so we can’t tell that video consists of tons of sequential/interlaced images, however, if we were to slow it down to 15fps or 10fps we would see lots of glitchy video. The first field contains the 540 odd numbered lines, because 1/30th of a second later, it is followed up with 540 even lines, completing the picture. So basically within 1 second of video in a 1080i brodcast, there are only 15 full 1080-lined frames, if they were combined (deinterlaced).
With 720p, you do not have the even/odd frame method of displaying the picture. Every single frame is a full frame, 720 lines. So the bottom line is that in a 1080i video you have 540 lines per frame, and in a 720p video you have 720 lines per frame. This is why the ABC network adopted 720p a long time ago, even when 1080i was an option, because when every frame is progressive you get a much smoother picture, on fast action scenes, sports, etc… So ABC made a smart move going 720p.
Interlaced vs Progressive clip:
Notice the lines that display very clearly on interlaced motion. The lines are there during the slow scenes as well, you just don’t notice them until a fast, high motion sequence, occurs.
Sidenote: Don’t be fooled by this new Dish Network TurboHD service. It claims “1080p VOD” (video on demand), but all it is, is deinterlaced 1080i signals, because no network right now is broadcasting in 1080p. Right now if you have a “1080p” television, the tv is just deinterlacing the signal to make each frame a full frame…that’s it. It is just a cosmetic gimmick, you are not really getting a 1080p video signal. So all TurboHD does it convert/deinterlace the signal on their end before they send it out…it is still a 1080i signal, the only benefit is that it will look better on televisions that do not have a built in deinterlacer (i.e. older LCDs and RPTVs). Most every new TV now has a built-in deinterlacer that can deinterlace any 1080i signal. Why do you think their packages start at $24.99? It’s nothing new, but I am surprised they haven’t been sued by other companies since it’s not true 1080p.
So, back to the main discussion…720p contains full frames, for every 30th of a second, wheras 1080i contains interlaced frames every 30th of a second. If you were to convert 1080i to progressive you would get 540p, which is less than 720p. If you were to convert 720p to be interlanced you would have 1440i which is more than 1080i. You could even go as low as a 542p video signal. Even that would be higher quality than 1080i, because it would equivocate to 1084i (if it were interlaced). Get it?
720p is not only more lines in any way you look at it, but since every sequential frame is progressive (full, not interlaced) then it handles motion and fast action flawlessly. While 1080i has to produce two frames to give you a picture, 720p just has to produce one frame to give you that same picture. I will glady take (and film in) 1280x720p over 1920x1080i any day.