He's got a 1366x768 screen
Outputting 720p means his TV will scale that upto native, so 720p to 768 some upscaling
If he outputs 1080i, it'll be 1080, but interlaced. Not to up on interlaced tech, but may have juddering, to do with 4:3 pulldown, or interlaced artefacts. Not sure.
Try both.
Would be ideal if your source has 1366x768 option that way native and progressive