When you asked about how good the "AV" is I assume you meant AF as in auto focus and not aperture priority since AF makes more sense in this context.
I own a 6D and although I don't own a 5D mark II I've used it a bit and all I can say is that the auto focus of that camera doesn't seem to bee on par with the 6D's. I also own the sigma 150 2,8, albeit the OS (image stabilised) version and it has no compatibility issues with the 6D or the 5D mark II.
I've tried to find comparisons of the weather sealing performance between the two, but it seems opinion based with no clear advantage to either camera.
Comparing the specifications the 6D has
- a better auto focus system
- higher resolution and better low light capabilities
- WIFI and GPS in body
- is lighter and smaller
- has the latest GUI and better screen
- SD card
The 5D mark II is by now outdated and only beats the 6D with
- higher maximum shutter speed of 1/8000th of a second compared to 1/4000th of a second in the 6D
- CF card slot (although not necessary better, it can be advantageous if you own a lot of these)
- it flash syncs slightly faster
- higher resolution
I would definitely not get the 5D mark II instead of the 6D unless the price tag was significantly lower.