You have to keep in mind that the difference between 10wpc of amplification and 20wpc of amplification is only 3dB of SPL headroom. Whether that last 3dB will be important to you depends a lot on the size of your room, the style of music you prefer and your preferences with respect to peak decibel levels when listening.
You really can't speak in absolutes here unless you make the assumption that the system will be asked to reproduce a wide variety of music - including something like rock, pop, or orchestral music that will definitely benefit from the added headroom - in something other than a small acoustic space.