How should I decide what branch to pursue in the Army?

At the end of the day, do what you want to do (that you think will make you happy). If being in the FBI/CIA is your end state, then perhaps sucking it up for a few years and going MI should be your goal. Have you talked to a FBI/CIA recruiter and asked them which branches bear the most fruit in terms of recruitability (ie is MI really the best branch for that)? Did you know you can go MI and get branch detailed to the Armor branch?

If you have skills would not the FBI or CIA hiring you now? Both have great training programs.
I would say you might want to learn to present yourself better, ROTC is normally capitalized. In the FBI, everything you write represents you and the FBI. Might want to start practicing. MI is also tough on people with poor writing skills.

Your class rank will determine what branch you will get. I wouldn’t get too tied up in what branch you get. First you’ll need to master your current career. There are multiple opportunities to work with other governmental agencies and give you the exposure you want. Have you considered branch detail first? You can spend your first four years in a Combat Arms Branch then when you make Captain, attendbthe MICCC and be MI. I had a friend do that and he was interested in the FBI or DEA. At the end Ms if the day, having a 4-year Degree, Military experience and hopefully a TS should really roundout your resume. The management, leadership and critical thinking skills will give you an edge when applying for any of the agencies. As a junior officer the majority of your assignments will at the tactical level. Once you make Captain you may be able to apply for more complex assignments with other agencies or seek positions at strategic HQ or the Pentagon. I’d seek a mentor who is in MI (MAJ-COL) and try to make a contact with the FBI now so you can customize your career to better serve your end goal. I know the enlisted jobs can seem more technical on paper, but the freedom and control over your own career will help you to achieve your goals in shorter time. Good luck.