I had a 2012 vehicle with Ford Sync, and it has BT audio, and now I have a 2014 vehicle with Ford Sync and it's essentially an improved version of what I had before.

I also have an iPhone 5, and my girlfriend has a 5S. We've used USB audio with both of our iPhones, but using bluetooth is a significantly better option for me as I can use sync for hands free calling, and the system will even read text messages out loud. I can hit one button on my steering wheel, say "call dad" and it will call my dad via BT. She has a car with sync as well, and prefers BT for the exact same reasons.

None of the extra features are available when you use USB audio through an iPhone, plus, because the USB port only outputs about 300mA, the phone charges at about 4% every half hour.

All of the BT metadata (Artist, track info) appears on every display in my car, it doesn't matter if I'm using my iPhone or Android phone. I can pause, and skip tracks with the steering wheel as well. The experience is exactly the same. In addition to added features like hands free calling and the ability to recite SMS messages, BT audio allows to me to a 2A charger to charge my iPhone or Android phone because the USB port is free. This is why using BT is a much better option for me.

USB audio is supported in 5.0 and above. I've personally used a USB DAC with my Nexus 5 and Nexus 7 and can confirm that it does work. The manufacturer of your car does not support USB audio though Android. That is the issue, and in my case using BT is a significantly better option, and USB audio is the inferior method. When using A2DP (BT audio) my Android phone can stream audio at 320kbps, and I can guarantee that 99% of people would not be able to tell the difference.