Im not going into the tech details but 1080i shows more lines per frame whereas 720p only shows 720 lines but refreshes them better (i think)
I use 720p for the x box as well as Sky. There isnt much difference between the two.
Try them both out and see which one suits you better. There is no better one as 720p sacrifices lines while 1080i sacrifices refresh rates
Also search these forums in the LCD section for loads of info on 720 vs 1080