Two subs are always better than one. They extend the soundstage.
If you are looking for realistic sound then the subs will be turned now to a whisper... if they are not then the sound field will become compromised and broken up by the extended bass interfering.
It really depends on what you are looking for... if those hip hop cars where you can feel them go by the bass vibrating your car... then go big. If you're looking for accurate sound then smaller.