Both A10 (hap) and BFE are correct when it comes to grad school, if you're interested. AFIT isn't a secondary grad school for engineers, it's actually more the primary. I know some engineering majors in A10's class (class of 2012) who graduated in the top of the class who went to AFIT for grad school. It's the air force's main engineering grad school. Not that some don't go to grad school at a civilian school. But I don't believe that the highest grads go to civilian grad schools and the lower go to AFIT. It's just a matter of numbers. When the Air Force has its own grad school, they can allow more students into it. Just like some still believe that if you don't get ENJPPT, you can't be a fight pilot. That's not true. Plenty of fighters come out of Laughlin, Columbus, and others.
Now, as far as the non-engineering grad schools, that is a horse of a different color. For the most part, you pretty much have to be a distinguished grad, and more importantly, be the tops in your department. (Your major). And these grad school slots out of the academy are much more competitive. Whether it's MIT, Harvard, Truman scholarship, grad school in England, or even the RAND, which I believe is the only direct grad school out of the academy offering a PhD; as well as some other, these are very competitive.
AFIT and these other grad schools are offered to only about 10% of the graduating class. So if you get one, great. If not, no big deal. You're in the 90% who will get their graduate degree either part time while doing your job, or you'll possibly get selected later to get an in-residency slot and go full time.
But as for you looking at what jobs interests you, there's nothing wrong with that. But as I mentioned, just about any job you look at, is available in more than one branch of the military. So concentrate on getting into the academy first. Yes, if you want to be a missile officer, working with icbm, the Air Force is probably your best bet. Or navy if you want to work with nuclear propulsion. Or army if you want tanks and infantry. But to an extent, all branches deal with Intel, battlefield management, cyber, telecom/It, logistics, contracting, special forces, etc. You'd be surprised at how many assignments dealing with "world wide" missions, have Air Force, navy, and army officers assigned and stationed together working together.
So, unless you have a very specific job in mind, that is branch specific, you shouldn't need to worry. As for what to say to your MOC during your interview on what your military career goals are, just don't be so specific. Meaning, you don't have to narrow it down to a specific job. You can mention working in the intelligence field, or with the newest technology, or even working in airfield/battlefield mission planning. No MOC is expecting you to know the specific job you want at this time; and they also know that based on military needs, the jobs may not even be open. They just want to know that you've thought about this decision to apply INDEPTH, and not just.... "Oh, the academy sounds like a good option for college". Best of luck.