What type of shooting do you do and what are you hoping will change?
The 5D II is an extremely good camera (I have a 5D II and III). You may gain a tiny amount of ISO performance but nothing that will blow your socks off considering how good the 5D II already is.
The focusing system is moderately improved -- still only one cross-type point, but 10 single axis points instead of 8 single axis points you have on the 5D II (so you gain 2 more single axis focus points which are left and right of center).
It does have GPS and WiFi. I don't think the build quality is quite as good on the 6D.
I've heard the complaint that the shutter button the 6D is "mushy" in that people have a hard time feeling the difference between a half-press and full press of the shutter. A friend of mine recently bought a 6D so I had to try it out. I do see what they mean by "mushy". On your 5D II there is a noticeable increase in pressure needed between a half and full press of the shutter button. On the 6D it's a marginal difference. This actually surprises me because Canon has been building cameras and shutter buttons for so many years that I have no idea why they would change it and.... IF they would change the feel at all, you would think they would only change it if the new button was noticeably better... this button is not as good.
Thanks for the feedback, you are correct the 5DMll is a great camera. I shoot mostly landscape and macro and the Mll does an excellent job in these areas. It just seems to me, that when Canon drops a model the updates and improvement are also dropped and I see very discussionvon the forums. I would like a 5DMlll but its not in the budget this year. I also use a 7D for action photography like rodeos, birds and other moving objects.
I think the main reason to consider that "upgrade" would be for low light, high ISO shooting.
The 6D is more than "a tiny bit" better... It may offer less than one stop (shooting RAW) or up to two stops (shooting JPEGs) higher usable ISO than you currently enjoy with your 5DII. This seems to be mostly coming from the software processing the images, to a lesser degree the new sensor. That suggests to me that you also might get some extra mileage out of your 5DII with updated software... Canon's own DPP now or 3rd party softares in the future.
The 5DII is rated to focus as low as -1EV. (Actually I think it's a little better than that. A 5DII can still focus, even tho it's slowly, after my 7Ds have given up, even tho Canon rates both models the same). The center point only of the 6D is rated to -3EV, two stops lower light, approx. equivalent to moonlight. That's a nice complement to higher usable ISO.
Not really earthshaking, but someone shooting a lot of low light might find it a worthwhile "upgrade".
Folks who have used both also seem to think the 6D's AF is slightly better with moving subjects, but that's "center point only", same as 5DII. I haven't heard comments about the shutter release, but haven't really been tracking 6D reviews very closely.
Aside from some additional low light/high ISO usability and limited improvement in action shooting capability, I agree and am not sure it would be very much of an upgrade... In terms of build, sealing, ergonomics and such the 5DII is still the better camera, IMO.
6D is definitely a nice camera and a good value, but considering what you use your 5DII for you would probably be wiser to just keep keep what you've got and continue saving toward a 5D Mark III upgrade in the future. That would give you more similar build and sealing, high ISO/low light capabilities pretty similar to 6D... plus a much improved AF system that's actually able to shoot sports/action, too.
I've been looking at going FF from a 30D, so I've been looking at 5D/5DII/6D.. (5DII is top of my list, seeing as I can't justify a 5DIII)
DxO do some good stats and they have the 5DII effective ISO at about 1800 and the 6D at about 2400, that means the 6D is only half a stop faster than the 5DII.. the in camera processing for JPEGs is doing the rest.
I think it is actually closer to a full stop. DxO is not reliable, I'm frustrated with how bad their information is, and wish they'd just get out of putting out bad data entirely.
In order to be considered a "serious" benchmark, one MUST DISCLOSE TESTING AND SCORING METHODS.
There really is no getting around this. DxO does not disclose and there's an interesting story where they allowed a writer to do a story on them. The writer learned that (a) they consider their scoring system to be their secret sauce... they will not disclose how they do that. But also (b) he learned they do a few things which effectively tamper with the data but then do not disclose to their readers that they do this. Apparently (just an example) when two cameras don't have the same resolution, they "resample" the higher res camera down to a lower res... claiming that this puts the cameras on equal footing. Unfortunately it also has the side effect of smoothing out noise. This means the noise scores really aren't accurate. They _really_ need to disclose this fact to be ethical... but they do not disclose it (they still don't disclose it... we only know it because they allowed someone to do a story on them and the writer brought it up in the story.)
I don't consider DxO to be reliable or reputable and I certainly don't consider any of their data to be a meaningful indicator of camera performance. I am convinced that I get more reliable data from Popular Photography and they take advertising dollars from vendors whose products they are evaluating.
Yes DxO make a fudge in their data for differing sensor sizes and pixel densities.
However the 5DII and the 6D are more or less equal on both counts.. certainly much less than half a stop.
Of course if it's absolute tech data then this is possibly the best report I've seen, but it doesn't cover the 6D, it could however form the basis of a 'reverse calculation' on DxOs numbers
edit: Comparing DxOs 40D and 50D and 5DII with the data in the above link does more or less line up.. The "low-pass-filtering" for differing Mpixies doesn't actually have a huge impact. (about 40% change between the 5DIIs 20Mpix and the 40Ds 10Mpix, far far less for more similar Mpix cameras) the real difference is making sure we've accounted for SNR rather than just quoting readout noise, where the light on the 40D is shared amongst far more pixels on the 50D vs 40D so most of the gains in low noise readout are give straight back in low signal strength per pixel.
Also they are trying to compare apples with apples by effectively giving the APS-C sensors a 50f1.8 and the FF sensors a 85F1.8 for the tests. The effect of which is to give the FF cameras an immediate 2.5x advantage, but that really does hold true in the real world.
Also, I've tested the readout noise myself on a number of astro cameras, and I've seen noticable unit to unit variation, so the discrepancies I'm left with between Buils data and DxOs falls well within the kind of sensor to sensor variations I've experienced.