It depends on which your 1080p set does the best job down converting. Try both. In all probability you won't see any difference between 720p and 1080i.
Well you made me check the difference by comparing the cable feed for 720p v 1080i. 720p won on my 50" 1080p plasma for the NBA game I was watching at the time -- the colors seemed more vivid.
I have a friend who is a certified ISF technician and has calibrated 1000's of televisions. I stand by his advise. At 50" or less there is no difference to the human eye between 720 and 1080. Larger than that and the difference MAY be apparent, or not depending on your distance from the screen.
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