FWIW Over a decade ago, my niece & my nephew (sister & brother) graduated Brown & Chicago as math majors. Niece is a college professor in math. Nephew worked at a Chicago based hedge fund for a few years before moving to NYC for Masters at Columbia & is now a published author. Father is a chaired professor so kids received strong guidance with respect to college selection.
Seems like both Brown & Chicago are great options for a prospective math major.
My thought is that part of the decision should rest upon whether one wants an intense versus laidback college experience since both schools offer strong math programs.