It recently occured to me after Nikon announced that with a grip the D800 shoots 6 FPS that I can't find a single advantage to the 5D3 over the D800. Here's what I know so far:
5D3 vs D800/E by catagory:
Dynamic Range: Inferior
Color Fidelity: Inferior
Color Fidelity? All modern dSLR's have excellent color fidelity; way better than film ever was. It's really irrelevant unless you're in the art reproduction market.
FPS: Equal (with a grip)
The "with a grip" nonsense is idiotic, IMO. Nikon's stuck in the 1970's with this notion.
We don't know that. Early tests seem to indicate Nikon's video is pretty mediocre. Early tests also show the 5D Mk III outputting 1080i video from the HDMI port, so that D800 advantage may be nothing.
Autofocus: Equal (both are pro level)/Inferior (no f/8.0)/Superior (better with fast lenses due to special f/2.8 sensors)
White Balance: Inferior
Auto Exposure: Inferior
We don't know that, yet.
In Camera Lens Correction: Inferior
You shoot a lot of jpegs? I didn't think so.
Pop up Flash: Inferior
AA filter: Inferior
Oh, you got that one backward. The AA filter is necessary, unless you like your images to be full of stair stepping, false color and moiré. Removing the AA filter is a very quick way to ruin photographs to the point of uselessness.
I realize that the 5D3 has a better menu system, so that's one thing it can possibly do better.
Don't forget its wider ISO range, access to the largest system of professional lenses in the world, and a service department that's second to none. Did you read about Nikon putting the smackdown on independent camera repair shops? This is not a very consumer-friendly company we're talking about here.
Don't forget the total cost of ownership. I recently did the math to replace my "L's" with equivalent Nikkors. It would have cost me about $1,200 more than it cost to buy my Canons; and I used Nikon's "refurbished lenses" site to get their prices.
Also please don't smite me. I'm not trying to bash the 5D3, as I'm actually a Canon fan and will be getting the 5D3, but can anyone think of one major feature that the 5D3 does better than the D800? If so feel free to post, I'm just trying to get informed here.
My sole complaint about the 5D Mk III is Canon's inability to get a handle on low ISO read noise and improve the dynamic range. I'm pleased with the new high ISO capability, and I love most of the other upgrades. It's what the Mk II should have been.