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.