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.

PSYOP is not the MOS you want to venture towards if you are submitting an officer package. The demands to learn fully your abilities and capabilities weigh heavily on time. By the time you have transferred from the Guard to the Army Reserve -- and then been sent to the 37 school --- that packet will have long been submitted and reviewed. If you opt to stay on the enlisted side - then it is an item worth pondering. There are no PSYOP officer initial entrance placements - you have to do a basic branch and then volunteer to transfer to 37 as a Captain.
SGT Carlos is on target with part of the focus - marketing and selling of the target audience are a central item. However - there is more to it than that as you work the specifics of the TAA and the individuals within it. The skill sets cover far more than you get in school - and its now in being a jack of so many trades with the platforms that communications are used from. The role of digital forensics is one of those items.. the advancements in that field spawn a lot of things.. But such is but one tool in a very big toolbag!

If I were you I would really look at CID. I know they don't have any officers but they have Warrants. You seem to have the qualifications for it. They are sworn federal law enforcement officers. They means more than anything else the Army can offer.

Don't over think it. Being a Military Veteran will be a feather in your hat but don't put too much emphasis on a particular MOS to be in the FBI. Regardless of MOS background you will go through the same path to becoming an Agent. Pick the MOS you want not the one you think someone else will want.

You really need to ask the FBI that question. Way back when I was a clerk in OKC, they had a list of priorities for hiring for Special Agents. There were ten at the time, and at the top of the list were degrees in law or accounting. That list has probably changed over the years. Call the Bureau's field office near you and ask about employment opportunities, or go online. http://www.fbi.gov. That will probably give you the information you need.