I'd just add that a direct USB connection to any decent, contemporary car radio will generally produce MUCH better sound quality than an AUX input, and might be significantly better audio than any Bluetooth streaming option.
The BT has to degrade, compress, stream, decode, then play, so it's absolutely lossy. AUX uses the DAC of the iPhone, which SHOULD be worse than a stereo's DAC, even in the lower-priced units.
I'm personally leaning towards one of these newer digital-only units (no CD player), just to do a direct USB link and rely on my iPhone's display. In fact, I'm kinda leaning towards getting an iPad mini as the "screen" of a setup like this, and just keep that iPad sync'd / logged in to all my main accounts. Will post updates whenever I pull the trigger on a plan
