That's all well and good, but my own eyes suggest that there's a difference. My cable box tells me when the signal is 1080i and when it's 720p and 1080i is often noticeably better to my eye than 720p. Sometimes, it's hard to tell the difference (likely because a quality 720p can look every bit as good as an overly-compressed 1080i), but whenever I go "wow" at the picture quality and look down, it's 1080i. Now, maybe it's just the fact that the TV's resolution is 1920x1080 and any other resolution on it looks worse, as is the case with computer monitors. If so, then there may technically be little difference in quality between the resolutions, but, in practice, there is, and that's the more important thing, IMO.
Regardless, it's a bit of a pointless debate, since there's 1080p content that will unarguably suffer in quality on a 720p set, so anyone who cares about image quality should be buying 1080p sets, anyways. Besides, the 720p sets over 40" that are available are likely previous years' models (since I doubt that they manufacture them anymore), so will not have the highest quality, the most features or the newest software and services. If you care about any of those things, you should be looking at 1080p sets.
if you compared side by side then yes its noticeable. barely. If you had a tv in front of you and said whats the quality it would be a guessing game 99% of the time.