I am not sure which theory I lean more toward, but the whole idea that the film was all a dream seems credible to me for some reason, for the reasons Pauln6
particularly pointed out above.
I think the ending was supposed to signify the one true constant in both the dream world and the real world -- emotion. That's what Cobb and consequently the audience feels at the end, and really the film was about overcoming grief, so I feel like it doesn't really matter if Cobb was dreaming or not. The whole film is about the idea that the act of inception literally translates to your real world, and Fischer's emotional catharsis occurred in the dream world, so why not Cobb's? There's a lot of speculation that the entire film was trying to achieve inception for Cobb, so that he could get over Mal's death, so that perhaps when he does wake up, he'll be truly over her for good.
However, I do see how the film points out to various interpretations. I don't think any of them is either right or wrong. You can either choose to believe Cobb is dreaming, or in reality, or any other interpretation you see fit (i.e. it was all a dream). The ambiguity and expansiveness that the ending implies is indicative of the limitless possibilities cinema and the dream world have to offer, so in that sense I think the ending was absolutely perfect given the intent of the story and premise of the film.