actually, all HDTVs are required to support 720p and 1080i.
p and i stand for progressive and interlaced. with progressive, the tv displays a new image 30 times a second. Interlaced is a form of compression where the image is split up into rows of pixels, and the tv only refreshes every other row at a time, but at a rate of 60 times a second. progressive is better, because interlaced video has a tendency to blur.
I would chose 720p, but it's really up personal preference and what you're doing (watching TV vs. playing a video game)