I watched most of the Stewart show on the Mall, and I only saw excerpts of the Beck show. So I will not attempt to compare and contrast the two, other than to observe that Stewart's backdrop was the Capitol Building, and Beck's was the Lincoln Memorial.
I am of the opinion that both have very short half-lives.
It is not clear to me what impact Beck's rally had on people. From what excerpts of his program I saw, he was preaching to the choir. And no "legs" since then.
The Stewart program was clearly well produced, and I did find it entertaining. Good turnout, some fine musicians, a few comedy sketches, yada yada yada.
It will be deconstructed by the usual suspects, whom Stewart excoriates regularly on "The Daily Show."
But my hunch is that it will quickly fade away as a moment on the Mall, just like Beck's has.
Martin Luther King set a high bar with his "I Have a Dream" speech at the Lincoln Memorial in 1963. Neither of these rallies comes close in comparison or importance.