To compare 720p vs 1080i, during a football game, look at the white lines on the ground, especially the ones vertical on the screen. As the camera moves, you can see interlacing artifacts in 1080i. In 720p, the artifacts are not there. You cannot get away from these issues at all. Broadcasters will be happy if you watch in 1080i as it saves them bandwidth. For real quality in sports, always watch 720p. Don't get fooled by the larger number 
---------- Post added at 12:06 PM ---------- Previous post was at 12:02 PM ----------
@solid_snake4rd: Not really, all channels (except a few movie on-demand channels) are 720p/1080i. The perceived visual difference between 1080p and 720p is not large enough for the broadcaster to justify the exponentially higher cost to record, store, and broadcast in 1080p. Its like 7, then 10, then 15 mega-pixel cameras. 5 mega pixel is more than enough for casual photography. Anything more when displayed on a screen is not visually better, so why go for higher mega pixel and waste storage space?
---------- Post added at 12:06 PM ---------- Previous post was at 12:02 PM ----------
@solid_snake4rd: Not really, all channels (except a few movie on-demand channels) are 720p/1080i. The perceived visual difference between 1080p and 720p is not large enough for the broadcaster to justify the exponentially higher cost to record, store, and broadcast in 1080p. Its like 7, then 10, then 15 mega-pixel cameras. 5 mega pixel is more than enough for casual photography. Anything more when displayed on a screen is not visually better, so why go for higher mega pixel and waste storage space?