The more recent date would of course be the more accurate one. As far as why the year is different, I do think you're being a bit of a pessimist.
I mean, who knows if they even had a publisher
in 2006, which would certainly make Campbell's unofficial estimation for a release year more conservative.
As for a delay, it could certainly happen, as all of us who followed Psychonauts' life closely know. but then 2008 could easily happen too. Some sites are even reporting the game to be out in late summer
, not December, which really isn't that insane when you think about it. That's still a three and a half year development cycle (assuming they actually started active development right after Psychonauts - probably not), which is longer than usual for most games. That's also how long Psychonauts was in development to hear Double Fine tell it - the progress on the game made in the first two years was essentially thrown out, and simultaneously the company itself was being built in the early years.
Bottom line: I would not assume a 2008 release to mean a rushed game. Brutal Legend has a lot of advantages that Psychonauts didn't, not the least of which is an excellent team that was fully in place from the project's inception, rather than being built along the way. Also, the game didn't even get dropped by Sierra yet.