As mentioned before, these specifications refer to the amounts of horizontal lines and the letter designation thereafter refer to the way those lines are scanned. Both are within the HD-domain and considered standards for HD signals. So which is better?
Interlaced scanning (i) - 1080i (1920 x 1080) delivers 2 million pixels 30 times per second.
Progressive scanning (p) - 720p (1280 x 720) delivers 1 million pixels 60 times per second.
This leaves us with a 6-of-one and half-dozen-of-the-other scenario. Both systems will coexist as there are pros and cons to both.