I’d focus mostly on nonacademic factors; both schools are extremely strong in math. My SO had to choose between the PhD programs at Berkeley and UCLA for pure math, and ultimately it came down to the specific professors he wanted to work with. (He recently finished his PhD at UCLA and has no regrets.)
For a math/chemistry combo, note that they’re in the same college at UCLA (Letters & Science) but separate colleges at Berkeley (Chemistry and Letters & Science). I’d investigate the curriculum requirements and how easy/common it is to double major between colleges at Berkeley.