Originally Posted by Electone
There really shouldn't be any difference if you TV de-interlaces 1080i upto 1080p.
But if it's too old to have an HDMI input, it probably doesn't correctly de-interlace ANY form of 1080i. And even a lot of the recent ones (most? was last year), unless the brand new models have improved a lot, will not correctly process film-based (24fps) 1080i by detecting 3:2 pulldown.
If the TV is interlaced (CRT) of course the 1080i is already good and doesn't need de-interlacing. If it's progressive (flat-panel), probably better off sending it 720p with Blu-ray, as if it doesn't handle 1080i correctly you'll only be seeing 540p (effectively, I think). Definitely use 720p if it's not a 1080p panel (probably not if no HDMI). (Note: This doesn't mean always use 720p for non-1080 displays -- certainly use 1080p
if it can be accepted via HDMI.)