I have a PhD in Physics and now I've just joined a strat house.
After finishing my PhD I had exactly the same dilemma than you and, finally I realized that, for me, MC was much more interesting than IB. In particular, MC provides you the opportunity to face intellectualy stimulant problems and challenges while IB is nothing but boring computer-based work (very well paid, but nothing much more interesting than implementing physical models in fortran).
So if you would like to keep the best thing of Science, i.e. the intellectual challenges, I'd choose MC.
It's up to you! Regards