Which MOS would be more useful and would open more opportunities for a career with the FBI, 31D or 37F?

To put it simply, 37F is more about marketing and selling an idea (mission objective) to your target audience. The legal aspect of what we learn has nothing to do with law enforcement but more about international laws regarding what we do. If you meet the qualifications, you’d be better off reclassing to 31B and submitting a 31D packet in 2-5 years.
However, if you’re already trying to commission you should pick a path and stick to it because commissioning and reclassing would obviously conflict with one another.

IMHO, Officer time would do you better than either. But enlisted time in the MP field would do you better than enlisted PSYOPs time. (If you were looking at CIA, and argument could be made for PSYOPs, but you said you are looking at FBI).

But if you are looking WAY down the road / long-term than I have a couple of BETTER options for you aside from going Officer (because you don't necessarily get what you want when you go Officer - you may end up as a personnel officer (S1) which won't necessarily help you out very much).

1) CID. Look at becoming a federal special agent before going to apply to be a federal special agent. It appears that you can apply direct once you have completed BLC (https://www.cid.army.mil/militaryagents2.html).
2) MPI. You would need to go MP first, but MPI specifically deals with investigations, which will give you a definitive leg up.
3) Human Intelligence. Learn how to do source operations and interrogations. VERY useful skills for FBI, and they dovetail nicely with your education.
4) Counter-intelligence. These skills are a PHENOMENAL fit with the FBI mission set.
5) Finance. Forensic accounting is a big part of what the FBI does. This is one of those skills that is often overlooked in law enforcement.

Options 1, 3, and 4 all have associated Warrant Officers - which would honestly be the best of both worlds - you have the technical skills for your resume, but you also have officer time RUNNING stuff. But again, that is talking long term - 10 years down the road before applying to FBI (at least).

Right now you really need to figure it out and lock it down, though. Trying to pursue too many things at once is going to cause you to do all of them poorly.

There's no PSYOP in the Guard.