I'm debating one way or the other on these and I've decided to let the dust settle because specs are just specs and while they are fun to look at they only tell part of the story.
Here's some food for thought for both sides:
Is the D800 is limited by the 36MP massive files? It has multiple modes (FX/DX/1.2 etc) and a small/medium/large resolution for each, so you could shoot FX (Full Frame) at 25MP if you'd like, and probably many will.
Nikon included the the D4 Metering in the D800, and has focal point metering, 5DMKIII doesn't (that said its not a deal breaker for me on the 5D)
D800 has Auto Focus up to F8, Canon is limited to F5.6, the D800 also has manual focus assist where apparently the focal point will light up when properly focused, I can't find too much info on this, and I can't find anything one way or the other for the 5D (the auto focus is a big deal)
Can't say much about low light on either one other then both currently appear to be great, we'll know this in a few weeks.
Lens, both companies have some great lenses and are competitive.
Ergonomics & ease of use - Supposedly Nikon has better ergonomics, lighter, etc. My canon has been fine and the MKII is easy to use the bottons etc., this seems to be more marketing then anything else. Ease of use, I've heard Nikon's are clunky and Canon's are great, I'm can't comment on the Nikon side, but Canon is easy to use.
Finally, the D800 is cheaper, it is!, now go look at their lens prices
All that said, I am considering jumping, yes Canon MAY
come out with a high MP camera, but what will they cut to fit the 3K target, or will it be another 1.5k? Canon does everything possible to make sure one camera doesn't infringe on another by cutting/not including certain features.
Also it seems lately Canon has been going after profits while Nikon is going after marketshare, it makes me a bit hesitant to believe Canon will ever build something that actually rivals the D800 in anything but MPs (In other words, I wouldn't be surprised if they put the 7D auto focus/19point system in their D800 killer).
Good luck which ever you choose, I keep bouncing back and forth