[Maarten]

Working voltage isn't the most important spec here as they are likely to be on a 12V or lower rail. Impedance and rated life are the most important. That said, a 25V electrolytic will often have a lower impedance than the 16V one since it has a somewhat larger active area. Within the same capacitor series, impedance is often given as a function of physical size, the larger capacitor having a lower impedance.