Hi Eva, thanks so much for getting back to me so soon! :)
So for me, whether I do a PhD or go into development sooner (or if another problem area that aligns with my interest comes up) mostly depends on what would help me do the most in the long term, so I'm pretty flexible and open to advice. My impressions of the MPhil are that it's great now, but it might have a low ceiling, and because my undergraduate background doesn't contain any quantitative training, it would make sense to go to Tokyo to focus on these quantitative skills through their economics/stats modules.
Obviously this would only be worthwhile if economics really has the potential to have more impact in development than a development studies degree! And if it justifies the two years of extra study.
Understood, thanks so much for the advice!