To everyone saying 670 SLI: I already have one 680 so this isn't really an option! If I was starting from scratch it would be the obvious option but I've had my 680 since release day and don't really want to side grade to a 7970 either.
I will probably avoid AMD next generation as I'm always nervous about alleged driver issues for crossfire. Obviously nVidia aren't perfect here either by any stretch but I'd rather take my chances with them.
It's good to see such a variety of opinions though. It confirms that it's not such a clear cut choice and I wasn't being an indecisive wet blanket