To answer your question of 5.4 volt or 6 volt:

6volt would be prefered so you get the additional torque and speed, but it depends on the brand of regulator you use. Most regulators will become passive if the source voltage drops below the regulated voltage. If this is the type regulator you have then go with 6v. Otherwise I would go with 5.4.


Passive(stops regulating and allows source voltage through the regulator when the source voltage drops below the regulated voltage.)