Whichever one looks best to you
1080i and 1080p will produce almost identical images if you're looking at very static images, like colonial-period movies or landscape shots. For having a lot of motion, 1080p will look a little sharper. This, however, is assuming identical signal coming into the television and near-identical processing.
The key is to actually look at the resolution on the display rather than the whole 1080p/1080i labeling. Some Hitachi panels will be listed as 1280x1080, indicating that you have fewer verticals.
It all depends on the set. I'm a firm believer that a good 720 set will look much better than most "cheap" (don't read that as inexpensive though) 1080p sets.