"Using the 70-200 F2.8 will get me there as well + the advantage of the ZOOM flexibility, but I won't get the quality of a prime."
It depends on your camera, the lens, your shooting style, etc. I have both the IS and the non-IS version of the 70-200. The non IS is considerably sharper especially at wider apertures, enough so to see the difference on a large print. You probably wouldn't see it on a 1Ds, but you certainly will on a 1DsMK2. The weakest point of the 70-200s are shooting wide open or close to it at 200 mm. They are much sharper in the 130-180mm range than at 200. In comparison tests that I have done with the 85mm1.2, the 90T/S, the 100mmMacro, the 135 f/2 and the 200 1.8, the primes are almost always visibly better at every compared aperture. However, the 70-200 at 135mm is quite remarkable, showing almost no difference when compared to the 135 f/2, matched f/stop for f/stop. The 135 is good wide open, but gets much better one stop down and is fantastic by 5.6. Llkewise, the 85 is usable between 1.2 and 2, but hits its sweet spot at 4-5.6. The 90 T/S is great at any aperture until diffraction start limiting its performance around f/16. The 200 1.8, which is only available used, is easily the best of all the lenses I've mentioned, being absolutely tack wide open and retaining that 'til 5.6, when it falls off rapidly.
When I moved to the 1DsMK2, I started noticing that the zooms weren't quite what I was expecting. That's when I started looking at primes again and doing a lot of testing. So, recently, when we were shooting a group of people with a 200mm, I chose the non-IS zoom, as I knew that we needed f/11, and that lens is sharper than the prime at that aperture. Of course, this was on a tripod, and if I knew I was going to be hand holding, I would have probably gone for the IS, but maybe not.
I don't think I've ever been as conscious of the effects of specific apertures on image quality as I am now after way too much testing. One thing is certain, on the MK2, the images really start falling apart if you stop down farther than f/16. It becomes a trade off for depth of field, but a rather nasty one at that. Here's a great link that graphically illustrates how diffraction affects different sized pixel sites.http://www.cambridgeincolour.com/tutorials...photography.htm