Much research has been done on 720p vs. 1080i..to the human eye, there is no difference. 1080p is a bit of a different story.
You know how you're sitting at home, and you're watching your local news, favorite TV station, or program? 90% of the time, it is in 720p. 1080i requires quite a bit more of bandwidth, for not that much of an increase in quality. 1080i is often just a sales pitch, as it isn't used very often, I've noticed. It's usually either 720p for television, and 1080p HDMI for wired products, i.e., Xbox, PS3, computer, etc. At least, that's the way it goes in my house.