Hi Dan,
I've got a IN78 my screen is a 84" and I sit about 11 feet away I recently borrowed a mates PS3 and I though the picture was stunning, I saw no screen door visible at that distance with the DLP. But I've also got a Sony VW-50 Pearl 1080p but I've not tried it with the PS3 as I'am trying to sell it (keeping the lamp hours down) I have had it running 1080p from a computer and the main difference was I had to be almost right up to the screen to see any visible pixles it was that good.
If you are going to use the PS3 as a Blu-ray player the one thing I'd recommend is to find a projector that will display 1080p/24 as the PS3 will output 24fps which gives a smooth display on movement.
So yes I would say 1080p will be a better option if watching films is your thing...