The advantages of smaller paintballs is that they will cost less (they are cheaper to build) and more will fit in your hopper at one time. Additionally, paintball guns will be more efficient and you will be able to shoot more paintballs without needing to refill your tank.
There are two primary disadvantages of .50 caliber paintballs. Due to their smaller mass, they will not travel as far if they are shot at the same speed as .68 caliber paintballs and they are less likely to break on target because of their lower mass and slower speeds.
.50 Caliber paintballs are not currently the standard size of paintball and most stores do not carry them, though that could change with time. For the near future, if you are just getting into paintball I would recommend getting a standard .68 caliber paintball gun. If, though, you are familiar with paintball and are interested in trying something different, check out a .50 caliber gun.
