Some statistics:
Concerning grad school in general, Bryn Mawr is ranked 8th in the country in terms of PhD productivity while Mount Holyoke is number 35.
PhD Productivity by Discipline
Bryn Mawr was ranked 26th as a feeder school for top programs in medicine (Columbia, Harvard, Johns Hopkins, University of California San Francisco, Yale), business (Chicago, Dartmouth's Tuck School, Harvard, MIT's Sloan School and Penn's Wharton School) and law (Chicago, Columbia, Harvard, Michigan and Yale). Mount Holoke and Bard do not appear in the top 50.
Top 50 Feeder Colleges
While all statistics and rankings should be taken with a grain of salt, Bryn Mawr seems to have the stronger track record in grad school admission!