Most Helpful Girl
I guess this depends on how you see success... is success money, fame or happiness? You can find happiness in either profession if you enjoy them, but fame may be short lived if you are a singer, as you are only as popular as your latest songs... longevity is extremely hard in the music business. If you are an actor, then your fame can last a lifetime because of just one performance. Both singing and acting can make a lot of money, but with music, too many people take a cut out of the profits. With acting you are on contract and get paid fixed amounts with only the manager getting a cut.
Most Helpful Guy
It is like anything you are successful in that particular field - With singing/acting I say it is a mixture of luck and talent that helps you make it.