I have noticed no problems with audio quality. Bluetooth does compress audio, but if you're already listening to mp3s (even 320kbps mp3s) it shouldn't be affected.
"Uncompressed CD-quality stereo audio uses a 1.411-Mbps bandwidth. [...] Bluetooth, for example, stipulates a maximum available bandwidth for A2DP (advanced audio-distribution profile) of 768 kbps. So, for high-quality stereo audio, it is necessary to use some form of audio coding to reduce the required data rate." from
http://www.edn.com/article/472081-Bluetooth_Sufficient_fidelity_even_for_average_listeners_.php
So bluetooth can transmit at 768kbps, far above 320kbps mp3. To be perfectly honest, I have noticed no degradation in audio quality over bluetooth from what I normally listen to (320kbps mp3s on pretty decent (audiotechnica studio monitor) headphones, although it needs some pretty serious equalization in the car. The Bose bass is really powerful and it sounds great, theres just a ton of it.
I actually think the audio sounds worse over the aux jack if I am charging my phone; the switched-mode power supplies used in phone chargers add a lot of noise to the ground of the phone. And the ground of the phone is the ground of the aux jack, so unless your phone and car have been designed with a perfect grounding system, you will get noise using the analog out of your phone. (and the two would have to be designed together to ensure this). My phone is an HTC evo so its not some el-cheapo crap, and it still has this grounding issue.
other solution would be to get a much higher quality phone charger, but they don't really make those and theres no way of knowing if one will be better than another until you try the new one.