A pellet is more more accurate than spherical BB, from my experience. The industry agrees as there are no spherical long range weapons. The aerodynamics are better, see Diabolo pellet.
Let's let the OP come back and respond before we offer any more information. We are still trying to determine if this might be a schoolwork question. Thanks.
Elyon, you owe me royalties for trying to infringe upon my avatar copyright. Expect to be hearing from my attorney.
There is one overriding advantage of one projectile over the other, but it's true that you should try to figure it out yourself and share your reasoning with us rather than just expect a pat answer. I will, however, give you a hint. Consider how real firearms work.