I have a canon 550d rebel and was considering the "jump" to FF.
The Canon 6d is in my budget. However the Nikon D800 is available (refurb) for $2,300.
I ask myself - why spend almost the same on the inferior 6d ? Why does canon seem to give less and charge more ?
The 6D and D800 are in different classes. You need to compare apples to apples:
D800 refurb $2300 + $450 grip = $2750
5D Mark III Big Value Inc Brand New (limited quantities) = $2499 + $275 grip = $2775
You need the Nikon grip to do 6 FPS fyi, otherwise you get a meager 4, which isn't a lot in the real world.
The 5D Mark III has a half a stop ISO advantage over the D800, a reviewers comparing raw files find that the raw files have equal amounts of base noise, but the D800's ISO is calibrated 27% higher than the D800's (meaning iso 1000 on the 5D III is equal to iso 1270 on the D800), the 5D III was also designed to repond better to noise reduction by having a more gausian distribution of the noise, so that adds another quarter stop of noise, after NR.
The 5D Mark III then has slightly better AF in many back to back tests (depending who you ask they are even though, as both are good), and tests have also shown that no zoom lens can max out the 5D Mark III over more than around 90% of the image area, meaning with zooms there will be no real world resolution difference, and with primes only a select few (EXcluding most Zeiss lenses and Leica lenses and only including exceptionally exceptional lenses) can outresolve 28 megapixels, and only between f/8.0 to f/4.0, so the nikon resolution advantage is slim or none due to the limits of most lenses.
The major real world advantage to the D800 is of course dynamic range for shadow recovery, and those megapixels for landscape shooters that use ideal setups and need to eek every last drop from the camera (but it's a smaller advantage than the marketing would lead you to beleive). There are of course other minor differences, but for the most part the 5D3 is a better journalist or portrait camera and the D800 a better landscape and studio camera.
For most work they cost the same and offer similar features.
The core Nikon lenes are generally more expensive too, if you ignore Canon's insane early adopter "tax", the 24-70mm II is said to go down the $1600, which is the same as Nikon's with higher quality and other than that the core lenes on Nikon are slightly more expensive for Apples to Apples lenses.
Hope that helps.