I looked at Brown University and here's what I've found.
Brown's Physics department currently employs (from what I've found) at least five theoretical particle physicists (their research includes string theory).
Compare this number to MIT's and Princeton's thirteen.
Now, only seven paper's from Brown's five theorists have been cited by other scientists more than 250 times. Only one of which was cited more than 500 times.
One of MIT's physicists, Frank Wilczek (theoretical particle physics/string theorist), alone has 46 papers that were each cited more than 250, 16 were cited more than 500 times. (He's a Nobel Laureate).
Acceptance Rates:
MIT 9.87%
Brown 13.53%
Princeton 11.68%
Berkeley (CA) 15.86%
I have given you the numbers. Which is the better school? That is up to your perspective.
Numbers were pulled from GraduateSchool Shopper.com and the INSPIRE (high energy physics) database.