I think that, intellectually, he runs rings around Brown, but then again, so does my dog. That said, I don't think Setti is ready. Plus, and I almost hate to bring this up, Setti has a young family and recently lost his Dad. Is this really the time to mount what would be a no-holds-barred Senate campaign? Setti is a young and highly capable guy, perhaps he can wait a bit.
Brown's aura of invincibility is cracking. Boston Magazine
http://www.bostonmagazine.com/articles/scott_brown_campaign_confidential/">has an article from former Globie Eileen McNamara that takes Brown apart on his book and Brown's revelations on his past child abuse. Brown's days as a sacred figure in MA politics are numbered.
There are a number of people who could challenge Brown. The biggest obstacle to a challenge is that huge campaign warchest Brown has. Whoever runs on the Dem side has to unite the party early and then fundraise like mad. It will be a difficult fight because of this money advantage. It is not nearly impossible though, especially since Brown will be running in a Pres election year with a very Dem leaning electorate coming out to vote.