The grass is NOT greener on the other side.
It may look like it from a distance, but when you get there, you can often find it is only painted on.
Friends of mine with D800's complain bitterly about their greenish coloured screens, the poor auto focussing, especially when you pick a point on ther left side as well as other problems.
I have a 5D3 (as well as a 60D with around 150,000 shutter actuations), and I've never had a problem with them at all. In fact, I am still amazed at what a great all-around camera the 5D3 is.
It really isn't lacking anything.
While the spec sheets and DXO tests may say otherwise, in side to side testing, you'd be very hard pressed to see any differences between the D800 and 5D3.
I shoot everthing from weddings, to parties, carpets, products,interiors, portraits, landscapes - you name it, and the 5D3 always does a great job.
I initially went with the Canon system, because of the their lenses and their lower costs compared to Nikons.
The quality is maybe slightly better with Canon, but as most of us spend more on lenses than we do on bodies, the variety, quality and cost of the lenses is what really swayed me to go Canon, and why I stay with Canon.
The D800 - as 99% of products - had some problems on release that were fixed later. Early users are often beta-testers.
This thing of D7000 and D800 having a poor AF is a legend. It's just less noob-friendly or noob-proof than most Canon models, but they work great.
No one says the 5D3 is not a capable camera, but the D600 looks pretty much like 90% of it for 60% of the price. If you shoot low-iso you even have some serious advantage.
And Canon lenser are not always better and always cheaper, by the way. When Nikon is not good in some segment, Sigma, Tamron, Tokina, etc come in to help you.
I switched to Nikon on release of the D800 and haven't looked back. People make a bigger deal than it actually is. If even I have to switch to sony or canon, I will.
this isn't marriage. they are just tools. get over it.