What difference does it really make as to what people say after all are you going to ignore the evidence of your own eyes if it doesn't match what you have read or are going to choose the setting that is more pleasing to watch day in day out?
First of all changing the broadcast resolution to 576p or 1080i requires the box to do some serious processing and it's doing it on a budget, the results are then sent to a HD display which may have to do it's own processing again with scaling/de-interlacing hardware supplied at a price point.
Even if someone with a PX600 said 1080i was better it may be that with your own personal settings on the display from the basic brightness/contrast to the more advanced digital processing (motion, noise reduction etc) would mean the result is different.
Do you have HD yet and/or are you considering buying a PX600?
The 'correct' answer is that its going to depend on which Display you have and if you can put up with your TV bouncing between modes when the box spits out various signal formats on HDMI.
For some folk up converting the 576 line SD signal to 1080i in the SKY HD box is going to be the best option - for other folk Deinterlacing the 576 line SD signal to 576p or Deinterlacing and up converting to 720p in the SKY HD box is going to be the best option.
For other folk the 'mess' their Display gets in when you have the SKY HD box set to Auto may dictate that setting the box to Output 720p or 1080i with all channels is the best option; even if it means the SD channels look a little soft compared to viewing them at 576p.
The other thing to consider (in my case anyway) is that I can only get the HD box to switch between 4:3 and 16:9 when set to auto. Personally I don't like watching a 4:3 broadcast "stretched" to 16:9, which is the only option when set to 720 or 1080. Other than that, I have noticed a slightly better output when set to 720/1080 when watching SD, but the difference is hardly noticeable and the benefits of the picture being in its true aspect outweigh this.