I think that Flake and PWP misunderstood the nature of my question. I own the 40D, 70-200 L 2.8 IS II, 24-70 L 2.8, and 50 1.4. I'm looking to upgrade my camera to the 5DM3 once it's available and then upgrade my APS-C to the 7D. I have several years of experience with this equipment (70-200 since it's release) and a great deal of technical knowledge regarding the features of my camera and lenses. Since I do not own the primes I mentioned I was asking about the performance of those lenses against one another. I understand my style and how close I'd like to be to my subjects.
Well, you already have some great glass!! And we still don't know what you most like to shoot
And this is kinda important since you'll be paying quite a lot for a decent L prime (they are all decent!!!).
Those three lenses you were asking about are different in many ways, most significant and important difference being their focal length... That's what you gotta choose first... If you really have no clue, you could start by sorting your photos with the focal length used and see in which range you mostly frame your shots. Then you can maybe start considering if you want to replace that focal length with a prime and if that will be worth all the investment
For instance, 70-200 f/2.8 IS II absolutely rocks, so unless you need the faster aperture of the 85mm f/1.2 L and you take a lot of portraits, buying it would be a waste of money that could be spent on other gear (tripod, flash, filters, batteries, cards, blablabla) or better, help you fund your 5DIII...
Also, you already have a 50mm prime, the f/1.4 version. Do you use that lens a lot? If yes, do you mostly shoot wide open? If yes, maybe you can consider the 50mm f/1.2 L, otherwise it's, again, a waste of money...
You got 35mm covered by the 24-70 f/2.8 L so again, do you shoot a lot around 35mm? 2 extra stops might sound very appealing but do YOU really need it? Is it worth all that cash for your particular case?
wickidwombat's suggestion is great! You could rent those primes and see which one YOU like the most. At the end, YOU are the one who is gonna dump all that cash for it
Please take no offense here, but I think you first need to answer your own questions. And I'm sure with all that gear, experience and technical knowledge you say you got you'll make a much wiser choice based on YOUR real needs
than anyone here making the choice for you based on his very own experiences.
I wish you good luck with your decision. You will absolutely LOVE any of those primes!!