I have TH-50PX60U, I suspect the 56" version should have the same spec aside from being larger. My 50" plasma is 720p/1080i. I watch all HD content on 1080i and it's awesome. I've seen 1080p set and I personally can't tell the difference in picture quality with 1080i.
I think you would do just fine with the plasma. I personally prefer plasma over DLP. I just don't like the limited viewing angle of DLP and the somewhat bulky set.