Converting from 1080p to 1080i and 720p

[Rover]

I have read a few articles/forum posts etc on the pro's and con's of 720p versus 1080i

However, if one starts off with 1080p I am not sure how Edius converts this to 1080i

If it doesn't produce upper and lower interlaced fields (with lower resolution) from consecutive "p" frames, but somehow utilises the whole frames instead, then is the resulting "Edius converted" 1080i better than 780p

[Rover]

I over simplified my question.

The whole story should have mentioned that I have a stack of 1080p footage which I have to edit and put on to Blu-ray discs.

A lot of people reckon that of the Blu-ray formats available, 720p is better than 1050i

However, as I am coming from 1080p, I was wondering whether the 1080i that I can output from Edius would be of better quality than if I was starting from native 1050i

[Rover]

Sorry -to be even more accurate the original footage is 1080/50p and the other figures should be 1080/50i and 720/50p