They are both good schools. Obviously you would save a lot of money by sending your daughter to Beverly. I think tuition at Brentwood is $25,000 plus

ouch!
You should visit both schools while they are in session to get a sense of the campus environment.
Since you are considering private school, some other neighborhoods close to Beverly Hills you might consider include Bel Air, Beverlywood, Cheviot Hills, Rancho Park, Westwood, Brentwood and Sherman Oaks, which although is in the Valley, it's just over the hill from Beverly Hills.
Other private schools to consider include Crossroads in Santa Monica, Winward School in Palms, Campbell Hall in North Hollywood, Notre Dame Academy (all girls school in Rancho Park), Notre Dame High School in Sherman Oaks, Oakwood School in North Hollywood, The Buckley School in Sherman Oaks, and Marymount High School (all girls school in Bel Air). These schools might be a better fit for your daughter since you describe her as an "average student." Brentwood, Harvard-Westlake, and Marlborough are elite schools academically and socially, so she might find the environment challenging and might not even get in.