1080P mean flash the whole screen in a single cycle. 1080i mean in one cycle it will flash even vertical lines and odd vertical line in other cycle. There is quality difference if you know how to spot them.1080p and 1080i are very similar. as the difference is that the pixels and vertical and horizontal in the 1080p and in the 1080i its different.(cant remember wads its like)
Conclusion 1080p> 1080i ( technically)