It may be worth noting that at that distance range, and those screen sizes, it is unlikely that the human eye could discern much difference between 720p and 1080p.
Please note, this also does not show in which way the OP will be using the PS3.
Example, I was at Audioholics earlier today on the PS3. On the 1080p set, just for quick browsing.
The 720p set, the small text is a little more blurry.
Granted with the browser settings you can change text size, but it is better in the higher resolution.
As well, to the OP, do you plan on hooking up a computer to the TV?
If yes, then 1080p resolution would be better for you.
If you plan on just watching movies and playing games, then no, there will be no real discernable difference.