Good question! Personally, I voted on the GAU-94; it has a higher fire rate with greater spread, which makes it easier to hit targets with it from a distance. In general, the GAU-94 offers an advantage of quantity over quality when it comes to cannon deployment, which makes it more practical in the battlefield in general.
Of course, the M230 has its uses as well. Unlike the GAU-94, the M230 has larger caliber ammo with a lower fire rate. If you can compensate for accuracy penalties at range, or get closer to your targets without risking your helicopter as much, the M230 is an excellent option for dealing with ground targets as as well. Considering the similarity between the two weapons in terms of tactical application, I'd say that it just boils down to personal preference at that point. Use your instincts!