Well, supposing that someone were to leave their PS3 on constantly -- folding@home or something -- the power savings could probably make up that price difference fairly quickly.
I remember reading an article on Wired that said a ps3 left just on idle (no games, anything) would cost over $200 a year to run. So if the slim really does use half the electricity or whatever, it might be a decent savings in a year or two if you leave your system on non-stop.
Well thats why for each their own taste. both operate the same. If you were one of those people who do leave it on all the time, its up to them.