which would have more power output?

 

RMS Power vs. Peak Power
The amount of continuous power, measured in watts, that an amplifier produces is called RMS power. The higher the RMS figure, the louder and cleaner your music sounds. When choosing an amplifier, the RMS rating is the power rating you should pay most attention to.

Also, keep in mind that some manufacturers calculate the RMS power ratings of their amplifiers at different input voltages. For example, an amplifier rated at 100 watts RMS at 12 volts can produce considerably more power than an amp rated at 100 watts RMS at the more typical 14.4 volts.

Stereo manufacturers often display peak power ratings on the face of their products. The peak power rating tells you the maximum wattage an amplifier can deliver as a brief burst during a musical peak, like a dramatic drum accent. The RMS figure is more significant.

That's from http://www.crutchfield.com/infolib/S...amps-glossary&

 

It all depends on the design of the amp's power supply.
If it's a regulated supply, the amp will output a somewhat constant power no matter what voltage you give it (between 10 and 16V)..

On an unregulated supply, the output power depends on the system voltage. Soundstream is one company that used to build amps like this. they'll have a spec for 12V and 14.4V on the same amp in their catalogs.

to so answer your question, you'd have to know more about the amp and it's construction. if everything else is equal, then go for the one that reaches rated power at lower voltage. usually that will indicate a little beefier amp than the other.
hope that helps.