Upload & Sell: On
I'm much obliged to be able to answer your questions with a REAL ANSWER from a REAL OWNER of both cameras. I had the 5d2 for 2 years, and sold to go all Nikon. http://www.flickr.com/photos/45249503@N02/
My flickr will show you examples from both camera. Most of the "sets" are from the 5d2, and newest photo streams are with d800. I shoot almost all landscape, so I'll let you know from that perspective. And I like street photog. First, the D800 is a much, much, much better made and better engineered camera than the 5d2. The AF alone is so much better its ridiculous -- and I never even had an issue with the 5d2 AF. I like Canon's ergonomics a little better, to me they're simple to learn and master. And you have saved settings on top dial.
As far as IQ, yes the D800 has significantly better DR. And high ISO noise is close, to me, at full resolution. I hear if you downsample the D800 to 22mm noise is 1/2 stop or so better than the 5d2. Don't expect drastic improvements in noise or IQ. It's just not there. I hope people don't lose their panties over this statement, but IQ after 21MP is very incremental IMHO. I switched systems because Nikon has better options at UWA lenses -- like the 14-24 or 16-35/4.0 (which I own). It's quite a bit better than the 17-40L I had.
I miss the 35L a lot, and haven't upgraded to Nikon's equivalent because I can't afford it and had other lenses to buy. For me, Nikon has released the 24/1.4, 28/1.8, 35/1.4, 50/1.8G (which I own), and the 85/1.8G (which I'll soon own) in the past few years and that tells me they're ready to become the dominant SLR company in annual sales -- a title Canon has held for a long, long time. Nikon's new, budget primes are all amazing! They blow away anything Canon has at the same price point.
I looked at the 5d3, but with $500 higher in price (at the time), same DR as 5d2, and nearly same MP as 5d2 (and from all accounts, IQ is nearly identical) it was an easy choice. I'd like to have BOTH systems, but it's really pointless bc this is just an obsessive hobby. I make zero $$ off my "work" -- if you will. As said, if you have a ton of L glass then stick where you're at. The 5d2 is still an unbelievable camera, don't get me wrong. I sold mine because the price was dropping quickly with announcement of 5d3. Had the d600 been announced, I would have bought it instead of the D800. It's incredible for the price -- I see some selling new for $1,600. Unbelievable. Get it and buy you the 50 1.8g, 85 1.8g, and 28 1.8g. Also, I have the 24-120/4.0VR and its very.very, very good. I love it. Good luck!!!