For 50 size I like 720p, cause the pixels are bigger and the picture can be more vibrant. That is my experience when I had my 720p 75u, a Panasonic model from a few years back. It was almost LCD bright yet still held it owns with black levels and shadow details with any 1080p out there.
I think you are looking at the 50 S2 Viera model right? The s2 is Panasonic mid range TV, I have a S2 and really really like it. Thank God, it does not have THX mode which I feel is highly overrated because of the dimness of the screen. THX is how your set is suppose to look in a movie like enviorment ( totally dark) But how many of us want to watch our sets in the dark all the time? I had a THX set and hated it.