Dua sold more but her cultural impact doesn’t come close to Megan’s imo. Being local or global is not much of a play to me in this era of social media, especially when streaming smashes are avoidable.
Megan's solo career is local to The USA, while Dua is much more global, so her.
As stated above, Meghan's impact is a lot more exclusive to America than Dua's. Plus, Dua served a whole album and a streak of hits.
It's not comparable to Meghan who wasn't that huge outside of