Ok so I'm looking at upgrading to a new 5D MK III hopefully this week and will be getting some new glass to go with, I am looking at the Canon 16-35L F2.8 or 17-40L F4.

I will be mostly outdoors getting landscape photos and I know F2.8 is not needed for this but I imagine I will be also using the lens for indoor and night time shots occasionally too, trouble is with the price of the F2.8 being twice that of the F4 ($750 difference) is 2.8 really needed on a 5D Mk III for indoor shoots when all reviews of the camera point to much better ISO perfornance, could you simply up the ISO once to get a similar setup to using a F2.8 indoors?

What do others think?