Aside from the obvious weight differences, each ball must be custom fitted to your hand in order to provide maximum control and comfort. In addition, the finish of the ball is very important. A very hard ball will not grip the lane as well as a soft ball. This is very important in controlling how much one's ball curves as it makes its way toward the pins. Finally, the core of the bowling ball is not necessarily uniform.
Some balls are heavier on one side, further promoting spin and curve.
Good bowlers recognize how to use the properties of their balls to develop their particular throw. The ball grips the lane, and, if thrown with a spin, will curve much more easily than a hard lustre ball. If a straight throw down the middle is more your style, a hard ball is more in order. It allows for less finesse, but more power. Of course, what really determines the type of ball to use is the condition of the particular lane which one uses.