I played with the 6D prototype at Exposure in Toronto recently. Personally I found the ergonomics far nicer than those of the 5D Mk II - and well tuned for one handed operation. The 5D Mk II is a bit like holding a brick if you have small hands.. The only draw backs I see are the 1/4000 shutter and the 1/180 sync. However lets be realistic, the difference between 1/200 and 1/180 sync isn't going to mean much (1/6 of a stop). And I find when I need 1/8000 I normally also need an ND filter anyway, which brings me down to 1/500 or something like that.
The shutter life difference could be an issue, but then you have to ask how many photos do you actually shoot? And how long do you plan to keep the camera before replacing it?
However for me the huge selling point of the 6D is the "silent" shutter! Assuming it works on the production cameras like it does on the prototype - wow is it quiet compared to the 5D Mk II! It I can think of so many places (events, street, ect.) it would be great to have that feature.
The extra ISO performance, and -3 EV auto-focus are also really important to me (if they work of course).
Other things to consider. The 600EX-RT flashes can only do high-speed wireless sync with newer cameras. Of course you can do this with pocket wizards flex things, but I think that's the more expensive option now.
And the 6D is a bit snapper to play back, and zoom images. Don't know if that matters or not.
It's arguable which actually has the better build quality and weather sealing. Have to wait for someone to take apart a 6D to know for sure.
Overall I'm waiting for the 6D. However if I owned a 5D Mk II, I'm not sure it's actually worth the upgrade, more likely I would sell it, and try to get a 5D Mk III on-sale instead - which is a clear upgrade in every possible way.
If I was shooting mostly at ISO 100-3200, didn't need the quiet shutter, and could live with manual focus when the auto-focus gave out in the dark, and didn't plan to get 600EX-RTs, I'd go for the 5D Mk II for the price and live with the less ideal ergonomics.
As for the WiFi and GPS. Well, you can use newer Android phones with a USB cable to control the camera, so WiFi for remote release isn't something I'd miss with the 5D Mk II. And if I wanted to Geotag images, I'd just get one of the many GPS tracker apps for my phone, and sync with LightRoom and time stamps. Yes having them is convenient, but it's not a deal breaker/maker in either direction. If I had the money I'd get the 5D Mk III without a question and forget about the WiFi and GPS.
And if the option of switching brands isn't too painful, look at the D600. I did, and liked it for the most part. However the live-view implementation in manual mode is painful to say the least, so I'm sicking with Canon for now.