« on: August 10, 2013, 05:19:49 PM »
Whether there's a "huge" difference depends on your standards, your eyesight, what you've photographed, how your looking at the photos (what size monitor, what size prints, etc.). The difference between a T3i and a 5DII (I owned them simultaneously for a while) in low light is significant and quite obvious (as it is between a better APS-C camera such as the Pentax K-5, which I owned before the 5DII, and a 5DII). The differences among 5DII, 5DIII and 6D are smaller, but still, despite that odd post you linked to at dpreview, noticeable and, to my eyes, significant - so much so that after renting a 6D and 5DIII I quickly bought a 6D and recently replaced my 5DII with a 5DIII. Partly it's a matter of noise, but not only that - I find that in low light the colours from the 5DIII and 6D are more accurate than those from the 5DII. As for which one to rent, you might as well go for the 5DIII with its superlative AF system (the 6Ds may be good enough, though, depending on what you shoot; both are better than the 5DII for AF).
At what point the need to apply noise reduction kicks in depends on what you've photographed, how you're going to end up viewing it, and your taste (e.g. I would rather keep some noise to avoid losing significant detail); I recently took some photos outdoors at night, hand held, with my 5DIII and with ISOs going above 8000 and didn't feel any particular need to add any noise reduction in LR. You might have concluded otherwise.
I'm sure the 2.8 zooms you mention would be just fine (I've used neither; in that range I would rather use a prime or two); but you might also be pleasantly surprised just how good the kit lens (24-105L) is in low light on any of these FF cameras, especially since it has the advantage of IS which, depending on your subject, may be more useful in low light than 2.8 (or, if 70mm is long enough, rent the stabilized Tamron 24-70 instead).