I was debating this decision myself and recently ordered the 2100's. TBH, after a ton of crawling forums and reading reviews, it seems to boil down to this: both headsets are essentially equivalent. The Logitech seems to have a slight edge in sturdiness, where the Corsair seems to have a slight edge in fault-tolerance (fewer issues with charging, dropped connection, etc.... but a narrow margin). Both are reported to have decent sound, as far as a gaming headset goes, but will pale in comparison to audiophile headphones. The jury is entirely out on 7.1 with both sets, as some folks seem to really appreciate it while others feel you should avoid it like the plague.
For me, the deciding factor was the earcups. The Corsair set has microfiber earcups, where the Logitech has that weird faux-leathery thing that seems more common. I can't stand the faux-leathery stuff, as it invariably gets sweaty and uncomfortable and doesn't breathe. This is a personal preference though. I think the best way to make the decision (if you are determined to go for a wireless headset, as I was) is by finding whichever of these two sets you can find on sale for a better price