Some people leave their HD Receivers at 1080i some change them to 720p, all I can suggest is try them both and choose which you yourself prefer.
Transformation.
Take a family picture with a digital camera at 1 MegaPixel, reduce it's size to 1/2 the original in Photoshop at 300dpi, then send it to a special company who will print it on a 4 feet high banner. Result should be deeply deceiving.
Same for Television. Source comes from 1920x1080, reduced by Bell Headquarters to 1280x720p and 60 images per second and uplink to the satellite. If you set your terminal to convert to 1080i, the terminal will then stretch the screen (540 pixels more x 360 pixels more) and send out 30 images per second.
If your TV is native 1080p, it will then try to convert interlace to progressive, missing 30 images per second.
If your TV is native 720p (displays 768 lines), it will downconvert to 720 and convert interlace to progressive... so the terminal did stretch the screen for nothing.
Since all channels are 720p on BellTV, isn't it a better idea to set the output to 720p on the terminal and let the TV do the work ?