To clarify you are not comparing 1080i and 1080p here. You are comparing 720p and 1080p
1080i and 1080p are the same 1920x1080. One is interlaced meaning only half the horizontal resolution is shown at any one time. Whereas with progressive all 1920x1080 is show with every refresh.
On a 1080p television you will see very little to no difference between a proper 1080i and 1080p signal assuming the deinterlacing algorithm is any good and doesn't just drop half the resolution to arrive at the progressive output.
Just because a 720p television accepts 1080i that does not turn it into a 1080 set.
As others have also pointed out the PS3 and all current Blu-Ray players do not do true 1080p yet as they work internally process as 1080i and then convert to 1080p. The same can be said for the HDXA2 HD-DVD from Toshiba.
Considering that most high-end 1080p televisions have much better quality scalers you would be better off with using the internal processed 1080i and then letting your 1080p television do the conversion from 1080i to 1080p.