Okay, I know this is sure to be controversial, but what the heck.
The differences between the 5DIII and the D800 have been the subject of endless debate on this forum. Now, Nikon seems poised to release a well-equipped D600 at a remarkably low price point if the rumors are true.
The 5DIII reminds me a bit of the 50D. The 50D was a great camera, but kind of a sales flop. It came out just as video was being introduced into DSLRs, but it had no video. Most 40D owners did not choose to update (instead waiting for a 60D that turned out to be the 7D...well I won't get into all that again).
Anyway the point being that there was nothing wrong with the 50D, but that a series of missteps, bad luck and poor timing combined to hurt the camera's sales.
I wonder if something similar is happening with the 5DIII. By all accounts, it is a great camera and seems to be very popular among its target audience: wedding and event photographers. But, will Canon be able to sustain 5DIII sales over the next three to four years? Is it $500 better than the D800? Or, is the D800 actually a slightly better camera at a lower price point? And, will sales fall if Nikon releases an entry-level full frame camera and Canon is forced to respond.
Now, before the Canon lovers and the Canon haters all go ballistic, keep in mind I'm not suggesting this means Canon is stupid or getting any part of its anatomy kicked or anything of the sort. In fact, making mistakes and learning from those mistakes are what makes a great company great. I'm just talking about one model in an entire lineup and suggesting that when we look back four or five years from now, I wonder if the 5DIII will be viewed as great camera that suffered from a series of unfortunate events.
I see your point, but, I really think this topic will need way more time and data to tell the story in full. For a while, those that were complaining about DR in the mk3 vs the d800 kept bringing the amazon top sellers list, highlighting the fact that the d800 was ahead of the mk3 in sales.
Well, the tables have turned - http://www.amazon.com/Best-Sellers-Camera-Photo-Digital-SLR-Cameras/zgbs/photo/3017941
mk3 at the 7 and d800 at 8. Which is pretty incredible considering that the top six DSLR's in sales at amazon are all under $1100. The fact that either are putting up these kind of numbers against lower priced consumer grade bodies is quite outstanding.
So while this could be a 'maybe,' we need to see what happens after a full year of these new bodies on the market.
Remember this too - its not all about the specs. Think of the free marketing both canon and nikon have - if the world at large see all of our images - us, the people shooting on the gear, and the difference is neglible in terms of IQ to the viewer (sad fact is, a whole lot of our hard work goes right out the window unless your clients are very IQ sensitive. The standard wedding or portrait client won't be as discerning. But we won't let that stop us from doing our best to make people say WOW!!!!).... If the world see's tons of WOW images, and the world see's taken on a mkiii, or d800, or the more generic, canon, nikon...if the world sees great images, the world will think I want a new cam...if the bulk of WOW is nikon, they buy nikon and nikon wins, same for canon.
Sum it up though...I just don't think any of us can make this call this early in the game. Give it a year, then revisit...