WRT to your comment about Bell being in 720p. I remember seeing ads about Bell PVR boxes handling 1080p and while I thought it would be automatic I check my box just now and 1080p option is avaialble in settings I could not see the difference when I switched back and forth.
Bell sends all channels to you as 720P, so 1080i channels are actually "downconverted" before they get to your STB. What you set the output of the STB to has no bearing on what actually comes to you. It's usually best to let the TV do the conversion, so I recommend setting Bell STBs to 720P, unless you view on-demand programming, which can be 1080P. Of course you can also do your own experimenting to see which output format works best for you. Here's the FAQ: