That's a pretty old question. 720p is 1280 x 720 lines (and most 720 displays are actually 768p @ 1366x768 ). 1080i is 1920x1080. 1080i is interlaced and 720p is progressive though. So while 1080i has more lines of resolution, 720p refreshes twice as often so it is technically the better choice. That said, it will also depend on the deinterlacer IIRC, so one or the other may look slightly better on your display and to compound that, it can be difficult for most to even really notice a big difference between the two.
