Pick ubc. With 100% of respect, frankly speaking, after your Chem-phd graduation, you basically will be jobless. US has very tough Visa and green card policy. if choose Brown, it will be so so so crazy to handle your OPT/CPT. Whereas in Canada, at least you can get Permanent residence without so much trouble. Then, you can settle down eventually with some occupations, e.g. real estate broker, convenience store owner ,etc.