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.