It is suggested that capacitors help the amps deal with multiple high current BASS hits in rapid succession. Batteries can not respond as fast as a capacitor in terms of delivering high current bursts.
That being said, a capacitor will not extend your play time. It may, or may not, provide better sound quality for heavy bass songs.
Put your money in batteries and high quality amps before putting it in capacitors. Unless you're going for looks. An expensive capacitor with a voltage read out looks cool.