If you were inputing 1080i both would have to be downscaled so they would look similar. Maybe the 1366x768 panel would look slightly better but I think other factors would be much more important in picture quality.
But if 720p is being input you might be better of with a 1280x720 panel if the screen can do 1:1 pixel mapping.
Dont get caught up in pixel count. If its HD ready then there are more factors that effect pq. If you are pixel obsessed then look at a 1080 screen of which Sony are the best affordable option before the expensive Pioneer 1080p plasma.
The demo should be native HD probably streamed from a PC or even these days a satellite feed or blu-ray player.
At home you are watching material that has been uspcaled to HD resolution but it was not created in HD so the DVD player and TV use an appication to fill in the gaps between 576i and 720p/1080i resolution.
The results can look very good depending upon the quality of the source material but native HD brings the screen to life