learn both. it's not like "pick a sword or an axe" to study as a ninja. dont think one is superior. don't think you can get away with only learning 1. grid is better for grids and flex for constraint and autolayout strategies. both have features the other doesnt, both have features the other does. it's case by case, and the right tool will manifest itself while your testing and trying against your real content.
biggest tip:
strive to understand both for yourself, make them both weapons in your arsenal of layout, you'll need them.