mp3 is an audio format.
Aux is just an auxiliary input or output for an external source like for an iPod or other device.
No difference in jacks on the head deck or cord.
Quality depends on how the sound is recorded.
I would take 2 12 L7's over a 15 any day! The 12's will hit harder in a sealed box and be faster than the 15.
Also adding a 1 cap will depend on what amps you have. I would go with a high quality amp and maybe a dry cell battery.
