When it just comes down to usefulness, BS is more useful imo and it really can do a larger dmg output. Here's why.
We all know noobs there days pick BB no matter what, so lets just compare the BS and GS a little bit. The only down side with BS is that it can backfire when you use it wrong and send your opponents flying, they may not open the parachute since they know their about to die anyway. You could also kill your teammates by accident, but who's so stupid to do that right? Do take notice that with BS you can also do significantly more dmg to vehicles than you would with a direct hit of GS.
GS proves it's usefulness when you know where your enemies are, but some sneaky gunners can drop down from buildings or commandos may want to give you a present, so unless you have a good grasp of the exact moment when opponents usually are in a tight corridor, behind a corner or if by some retarded luck all opponents are in a really tight group BS will not work so well.
GS can only deal direct dmg, GS can be easily avoided, either by taking quick cover ducking behind a fence or something, either by BS form other soldiers or just your friendly neighborhood gunner eats them and becomes even more of a thread for you. Ok lets recap. BS, instant, more dmg, larger area, works against vehicles, protects from explosives, great for sneaky commandos. GS, clearly visible when you activate skill so can be prepared to avoid, can be deflected or eaten, needs specific conditions or really dumb opponents.