Either school would be able to deliver a fine undergraduate education in chemistry. I’m sure that both schools send lots of students into chemistry grad programs.
Smith probably has a bigger chemistry department, because it has roughly twice the enrollment of Bryn Mawr. On the other hand, Bryn Mawr has a very close relationship with Haverford College, which is is only a few minutes away by shuttle bus. It would even be possible for a Bryn Mawr student to major in Chemistry at Haverford if it seemed like a better fit. The combined resources of Bryn Mawr and Haverford are probably at least equivalent to those at Smith, if not larger.
Smith is part of the Five-College Consortium, so Smith students can take classes at UMass, which might offer more specialized coursework in chemistry than a liberal arts college. Similarly, Bryn Mawr students can take classes at UPenn. However, neither of these options is as convenient as the Bryn Mawr-Haverford option.
Bryn Mawr, while primarily known as a liberal arts college, is authorized to grant master’s and doctoral degrees in a few fields, including chemistry. So Bryn Mawr actually has a small graduate program in chemistry. I don’t know if Bryn Mawr undergraduates have the opportunity to take graduate-level courses in chemistry, but it might be worth looking into.