Whatever the one you want to do is, but you're not sure because of the other one...ignore the other one, do the one you want to do.

This is my take, as well. "Work with the smartest people" has been said a lot here. But from my experience, you can hardly go wrong if you follow what you are most passionate about. Build things you want to build, build things you want to exist. If the field you picked seems like a bit of a dead end, there's usually a way to pivot into something valuable. Maybe I just got lucky that this approach worked for me, but I think life is too short to not prioritise joy in what you spend most of your waking hours doing.