1080p is the best on HDTV's! But what's better between 1080i and 720p!?

Answer 1

All TVs that are NOT 1080p TVs can accept both 1080i and 720p signals. Since in most cases they have 736 lines of pixels, they will upconvert ANY input signal to display at 720p; what is called their native resolution.

If you have the option, It is up to you to decide if you want source signals sent to the TV at 1080i or 720p. Some TVs do a better deiterlacing/scaling job than the source (say DVd player or set-top box) and some sources do a better job than the TV. Just try both.