I've shot all my wedding work for the past three years on a pair of 5d mark II's and they're great cameras with very few shortcomings, those that it did have all been addressed by the mark III.
For me I didn't like, and tried to avoid going beyond 1600, but was happier still at 1250, more for the blacks getting clogged up and the files generally being less mailable in post production. This would be less relevant if you intend using a lot of flash obviously, but I usually avoid it if I can. A usable 3200 such as the mark III's plus the AF improvement to me were worth the purchase, although initially I'm going to work with a mark II and a mark III, the mark II will not be at any disadvantage for 80% of the time, so I'll wait for the price to drop before moving over to two mk III's.
Other advantages on the newer camera not yet mentioned here, it has a fantastic silent shutter setting, it limits the camera to three shots a second (I never shoot at that rate anyway), but is so much quieter. The hugely improved customisation is great, I have a setting that takes me straight into a user defined magnified review which I'll find very useful. It's so easy to set up how you want it, and at a pixel peeping level the files are sharper or perhaps just better defined, but they definitely have more pop on screen, but I haven't had any prints made yet. Auto Iso setting is actually useful now, on the mark II it was a waste of time.
With regards lens speed, I can only Speak of the lenses Ive used, but I'm on the cusp of buying the 50 1.2 having used the 1.4 for many years but don't tend to use it in low light because of its poor focusing which I've really tired of. I tried the Sigma 1.4, better build and AF, but very soft wide open despite what I'd read in reviews, and I tried two of them, both the same. So that leaves me looking at the overpriced 1.2.
The 35 1.4 is fantastic and not terribly heavy, but I never tried the f2 version. I did have the 85 1.8, a very competent lens, the 1.2 is both expensive and incredibly heavy, but what a lens, it has the most beautiful quality and is my most used lens during receptions, one of Canon's best.
This is just my take, the mark II is a great camera and I'd probably prefer it with a good lens than the mark III with a poor lens, and in close to 100 weddings with the mark II's, I remember two occasions where I've switched the camera off accidently because of the on/off position, and that's carrying two of them.
