1080i has looked the best on all the Dish HD recievers I've had. (my Samsung OTA STB too.)
I previously had 811's, and currently have a 942 and a 622. 1080i looks much better on all three of my HD TV's regardless of the connection (HDMI, DVI or component).
One is a 51" 1080i rear projection CRT, one is a 50" 720p DLP and one a 720p LCD Front projector.
720p looks like crap in comparison. My TV's are three different technologies and three different manufacturers so I would assume the Dish boxes(Like many Boxes) don't do a very good job of 720p conversion.
THe 720p output from my HTPC looks fantastic on both my 720p displays.
I also Had Charter Cable for two weeks and the 1080i output on their MOXI HD DVR looked much better than 720p as well.