What would benefit you in the long run? Why not both?
I think it would depend on how many classes you are taking and what level. If you are starting out in college and you are just taking a class, for a 8 week period, you would have time to study. If you combined your class with your study for the board, like American Military History, then you would have a good fit.
The one major item is what is the board looking for in an NCO? That target seems to move year to year. Look at the profile of your peers, do most of them have a two year degree? Or are they getting promoted with some college. The next question is what are you going to do after the military? That is the bigger question.
Boards are wonderful motivators. It's gives the soldies, sailor, airman, marine something to shoot for in competition within your unit.
I've never seen anyone hired outside the military based solely on that former SM having been designated as a "winner" of a board. Unfortunately, there are very, very, very many ex-military and only very few board "winners". Earning that college degree displays the ability to take a mission (get that degree) and see it through. Most employers don't have a clue what work is necessary to earn that soldier of the year award, but they certainly know what that degree means.