i'd go with the 6D over 5D2. the new features are nice (GPS!!, sensor, processor) and i think the camera is lighter. i haven't shot with 6D but from what i understand the AF system is similar to 5D2 and inferior to what you have on 7D. since you don't need AF tracking, you probably don't need to fork over another $1k for 5D3. the latter is an awesome camera and a significant upgrade over mk2, but it may not be worth the money for your purposes.
Yeah, I'm still up in the air about it.
The GPS isn't that important to me (though I suppose it should be) - the sensor and processor are driving it. I becomes a trade between the built-in WiFi and price on the 6D vs the AF, Raw HDR, and dual card slots on the 5D3.
There is no comparison between the 5D2 and the 6D: between those two, the 6D wins hands down.