I have a 5D Mark II. I bought it a few months ago for $1500 on craigslist. After owning it for a short time, I now know that I should have made that purchase years ago. I have a considerable amount of money tied up in "L" lenses. I've always spent the money on glass first. That being said...
If I were upgrading from anything less than FF, and could afford it, I'd probably go for the new technology. The 6D has Digic 5+, built in Wifi, silent shutter mode, GPS and a smartphone app. That alone would sway me towards the 6D. Though, imagine the used price market on the Mark II when the 6D hits. It would be very affordable to pick up a nice used Mark II for backup or whatever.
So, in the end, you can't go wrong with either choice. I absolutely enjoy my Mark II. This is a total win-win...
Scott