The main difference in our systems is the amplifier used
We use top of the line in all of our systems, and  Amplifiers from Cerwin Vega

Basically we have two speaker systems or 4 speakers systems. 

4 Speakers is going to be louder and better at highway speeds, but 2 speakers is very good and we sell alot more 2 speaker systems than the 4 speaker systems. The 2 speakers can be upgraded to 4 speakers by adding an additional set of unplugged speakers, and using the original amplfiier to power all 4 speakers. 

We use the same top of the line Kicker KSC404 speakers in our top end systems, so the major difference is how much power the amplifier give and the actual amplifier that is used. 

In short
More speakers is usually better than more power
More power is better than less power
All of our systems are really good so no matter what system you choose it will sound great