PhD Decision Science/Operations Research vs Finance

Edited to add details: I want to pursue a PhD because I wish to get back to academia and become a Researcher/Professor.
During my undergraduate, I enjoyed the process of experimentation, modelling, testing hypotheses etc. However, after my Undergraduate degree, I decided to take a break from academic life and started working in Financial Services Industry (though nothing I-Banking etc.) I will have completed 2 years of work by Fall 2019 when I'm planning to go back to school.
Now, I wish to go back to academia. The two fields that really interest me are Decision Sciences (mostly operations research in the field of optimisation of supply chain) and Finance (exploring impact of new trends/fads on short and long term industry returns etc.)

I have an excellent GPA (1st in Class with 4.1 out of 4.3 from a top university in Hong Kong) in Mechanical Engineering and a GMAT of 760. However, for want of enough Finance/Math research experience, I believe getting into Top 20 Finance PhD programs would be tough. However, given my background in Mechanical Engineering, I feel getting into Operations Research would be easier/more natural path for me.

So, in such a situation, should I go ahead with a Top 20-50 Finance PhD Programs or rather pick a Top 5 Operations Research program and work on my application for that? Which one would have a better career prospects (in academia, and in general)?
Or are there any Research Masters in Finance which can help my cause in getting into a better Finance PhD program?

Thank You.

PS: Apologies for not making it clearer initially. This is my first post here, so did not want to make it too large.