Weatherwise, Shanghai can be uncomfortably hot (and humid) for walking around, and it's also very polluted.

However, I'd pick Taiwan just because you know some locals there who will not only host you but also show you around. That can be priceless. Not many travelers are even privileged to see local homes, nor to get insight into lifestyles from someone who lives in the place. Personally, I would be fascinated to discuss comparisons between the USA and Taiwan academic/research systems, etc.