You aren't going to see much difference between 1080i and 720p. You will notice a difference with 1080p.
1080i, yes, technically displays more pixels - but not as many as you'd think because it is interlaced and not progressive (1.037 million for 1080i, .922 million for 720p and 2.074 million for 1080p).
720p, because it's progressive, will resolve artifacts better than 1080i during motion sequences. You'd really only see any sort of difference during a stationary shot with 1080i.