But knowing that my photos could be sharper or less distorted really annoys me.
Or maybe I'd be perfectly okay being stuck with a Rebel for a year knowing I have the best glass available.
You can fix distortions nicely with current software like LR and esp. dxo for complex distortions, and for sharpness, well, you know it depends on the final output size but I also feel that a shot should be sharp at 100% crop just for the sake of it.
If you're not going to sell your shots a rebel should be fine, I'm also still on 60d with the iso800+ iso problem.
I'm indecisive, aren't I?
I sympathize, because I know how hard it is to make up one's mind if top gear is out of reach for the time being. The one thing you should make up your mind about is if you really need the f2.8 of the 24-70, if not you're immediately saving €1000+ for other important non-"core" stuff like lighting accessories and filters. Btw - I'm also still undecided between 24-70 and 24-105 because of the immense price difference and IS.
The larger apertures imho are mostly for available light shooting and subject isolation if you want that, with flash support f4-f5.6 should be also just fine with the higher iso capability of the 6d.
Save $$$ until you ready, sell your 24-105 and get 24-70 f2.8 II
... or the 24-70/2.8IS when it's out :-)