I had recently upgrade to a new canon 5d Mark III . I was using the Canon 5D mark II before.  This is what i think

I had always use canon . I first switch to digital when canon released the canon 10d back in 2003 .

This is the first model in the 5D series that Canon has actually built to reasonably target its intended users, so well done there.

I think it is extremely annoying that they intentionally continue to withhold greatly needed productivity features: No voice memo, no built-in GPS, no built-in flash . Some time you still need the built in flash.  No A-DEP mode to assist with accurately controlling depth-of-field.

The 5D3 is better than the 5D2. But just not good enough for 3 years
What i like:

Finally, for the first time, we have a reasonable autofocus system.
Canon has also added some helpful usability features, so thank you for that. It’s quicker now. I do get more accurate in auto focus. My images are less blurry.

What i don’t like.
The low ISO files are just not good enough. I was expecting a lot better. They don’t respond nearly as well to adding fill light and general adjustment (in post processing) as do the files from the D800. I expected Canon to at least keep pace with Nikon in this recent round of generational upgrades. But I have been sadly disappointed. From all my testing over the past week, I am pleased with the files coming from the D800 – a clear generational improvement. And I am sadly disappointed with the files from the 5D3 – better than the 5D2, but not good enough. Up through ISO 6400 (the quality beyond that is unusable in my kind of pro work), the D800′s files are cleaner, smoother, less noisy, more adjustable, AND higher resolution than those from the 5D3.

Here are bold point for you to carefully consider:
1) The 5D3′s autofocus system is slightly better than the D800′s – faster and out closer to the corners of the frame. 2) Canon has a generally better lens lineup – generally optically slightly better and more comprehensive.
If overall ImageQuality and adjustability is king with you, AND Nikon has the lenses that YOU need, then go with the D800.
If you don’t adjust your files much and generaly use them straight out of the camera and pretty good quality is good enough. And if overall camera speed is critical to you (as in weddings), then the 5D3 is probably your best bet.

If you are in the wedding business, The 5D mark 3 would probably make a better wedding camera than the D800 because of the iso . There is almost no grain at 1600 ISO. But canon 5d Mark 2 you will get some at 1600 ISO. But don’t expect high ISO miracles, as the 5D3′s files are just not that great. They are more “smoke and mirrors” (fancy noise reduction) than an actual true improvement in IQ.

I also like the fact that Nikon give you to ability to switch to crop mode without changing the body like Canon.  I’m currently using canon 70-200 F2.8 IS mark ii . I have to switch this lense to a crop body ( my canon 7d ) if i wanted to be at 320 mm. For my line of work, sometime i stand far from the bride so i wanted the ability to zoom in.