it depends. both will be 720p in reality on an HD ready set, due to its native resolution (and the fact that they re natively progressive-style displays, no interlace). 1080i is more suited for detailed low speed content, so the de-interlacing crap doesn't show, whereas 720p is more suited for fast action. In reality 720p actually "looks" and "is" better since it matches the panel's ability 1:1 on HD ready sets. FULL HD sets on the other hand have a native resolution of 1920x1080 progressive, so you can imagine the ups and downs (everything is essentially scaled to 1080p). Additionally allow me to say that 1080i does indeed look better than 720p on actual PC CRT monitors - some monitors can do 1080i fine since they have no restriction on the number of scanlines (by definition, though some monitors are fitted with hardware to limit non-viable resolutions) - and as a bonus they can handle 50hz and 30 hz just fine (some of them) =)
