Just to get back to the question, the difference between 1080i and 1080P may or may not be perceptible depending on what material you are watching, what type of display/front projection you are using, screen size, and seating distance from the screen.
At one extreme, if the video material is static, lets say a still picture, then 1080i would be equal or better than 1080P. In other words, it doesnt matter whther screen frames are interlaced or deinterlaced because what you see never moves.
On the flip side, if the video material was in constant fast motion, you might see a difference and 1080P would provide, potentially, less blur and more clarity.
If your display cannot natively display 1080P material( ie its not a 1920 x 1080 display), then the difference is a probably nothing, as downscaling wont allow 1080 material to be displayed at 1920 x 1080, due to a lack of display device resolution( or pixels in the case of LCD and plasma).
Smaller screens and further seating distances would make the difference between 1080i and 1080P imperceptible in most instances.
Also source material native resolution makes a difference. You cant view 480i or 480P material at 1920 x 1080 i or p and expect anything better than the original resolution, unless there are qualitative weak links in your technology set up somewhere ( this happens).
So 1080P is "better" than 1080i when you are viewing 1920 x 1080 source material ( like Hi Def DVDs) that have a lot of motion on large screens essentially.
everything else gets a bit fuzzy but if you try 1080i vs 1080p viewing at a reasonable distance ( say 9-10 feet), you most likely wont see much of a difference.
