I would try both with some HD material and see which looks best. If it's an LED-backlit LCD display, I'm guessing that it's a 1080p panel? In that case, outputting 1080i would potentially be the best choice, but it depends some on the way the box behaves and how the TV's video processor works. If it is a 720p display, in some cases the 1080i output could be better.

If you have an option for outputting the source resolution all the time (480i being deinterlaced to 480p since HDMI doesn't support 480i), then HD sources would go to the display at either 720p or 1080i depending on the channel and recording. That would put the bulk of the video processing duties onto the Sony TV (or the surround processor's video processor, if you add an HDMI-equipped processor with internal video processing). If your cable box is like most of the ones that I've used, its video processing will be inferior to the Sony TV's processing.