What is the difference between using an auxiliary cable and a USB cable?

Aux connections tend to transmit background noise, plus you have to control the tunes from the phone itself — not the safest thing when you’re driving alone. The USB connection on the other hand, transmits no background noise, gives you receiver-based control (some applications) and also charges your phone. In many cases the receiver also processes the sound, something it’s designed to do better than your phone in an amplified system.