A lot of people consider 1080p to be the true hi-def standard with the other options used ie 720p and 1080i as being inferior.
Unfortunately, it's going to be quite some time before 1080p broadcasts are made due to the large amount of bandwith they take up so the majority of HD sets support 1080i as a maximum.
I believe that the bluray players due later this year and also the new HD DVD players that have just gone on sale in the states support 1080p. So if you really want the best quality picture and can afford the massive premium on a true 1080p tv then thats the one to go for.