It's not the same. In terms of resolution 1080i is definitely higher. But it takes 2 frames to complete an image in video. So if the image is slow or still, you will not notice the difference in image quality to 1080p. But in sports, u will see 'jaggies' as one frame has a ball in one position and the next half a distance away. Resolution of 1080>720 be it i or p.
If you deinterlace 1080i, it would be similar to 720p, right?