Visually with today's technology there is zero difference between i and p and 720p is no where close to 1080i sorry to dissapoint you.
The technology to broadcast in p isn't out yet. Everything HD is broadcast in an interlaced format, hence "i" and nothing is actually produced in progressive yet, which is "p".
TO original poster, go with LCD, yes there is some ghosting and blockiness with plasma nut most people can't see it. Never buy 720p, it's a scam to sucker in broke people.
-Matt
Nvidia Embedded System Engineer for mobile video technology.