I have spoken to a few friends who own the 5D3 and while it is a wonderful camera, they are still torn between it and the 5D2. Like most cameras, even though the 5D3 was meant to be a replacement/upgrade to the 5D2, there are still some things the 5D2 does better, or just as well.
If the 6D does come out and it has a lower build quality than the 5D3, I would say stick with the 5D2. They are going to continue to drop in price new and used. I love my 5D2, it takes amazing photos. Honestly, the $3400 for the 5D3 is keeping me from "upgrading". That's way more money than I want to spend considering I have a 21mp FF camera at the ready. It will be interesting to see what Canon considers as a "replacement" for the 5D2. I still believe an affordable FF body is needed and the 5D2 could use some upgrades, but will the upgrades be worth the price if you already have a 5D2???
Not to get off topic, but I owned the 5D Mark II and now have a 5D Mark III. Excactly what does the 5D2 do better? I cannot possibly think of ONE thing it does better.
I've heard people say that the 5D2 centre AF point is better than the one on the 5D3, and that the AF works better in low light on the 5D2. Not tried a 5D3 yet myself, so only going on what I heard.
In general, it seems like the new AF system is a lot more complex, and you need to really understand it to get the best results, where as the 5D2 system is pretty straight forward and perfectly capable for wedding photography etc.
Also, the 5D3 sensor is hardly a huge leap forward (in terms of RAW high ISO performance), so there's not a huge benefit in upgrading unless you require the FPS upgrade or the extra AF points.
Let me put it this way, it's a costly upgrade, and as a pro, it can only be justified if it's going to improve my work significantly, which I don't think it will. The new radio flashes seem like a much more useful upgrade TBH.