It's in the real world, not a dream.
I don't think that Nolan ever came out officially, but I do remember him "pretty much" saying that it was real : The point isn't supposed to be the spinning top, the point is supposed to be that Cobb turns away from it. He's back with his children (Who were played by different actors and were in fact older), so he didn't really care anymore. All he wanted was his family. That's what Nolan wanted people to "get".
I'll try to track down this interview if I can.
nolan NEVER said it was real. if anything he implied that the whole thing was a dream and actually stated that the entire MOMBASA scene was manipulated to confuse people if its reality or a dream. he repeatedly stated that cobb's an unreliable narrator... the ONLY thing you're correct is that the ending doesn't matter as long COBB accepts it as HIS reality, which was all he cared about..