AUX in vs USB connection clarity?

[Blood1]

I was in Bestbuy in the car radio section for hours today trying to decide which head unit to get. I finally decided to grab the 3.5mm cable from my car and hook it up to one of the units i was interested in. I was pretty shocked to hear the different between the sound and clarity over the unit's radio versus the sound coming from my Sansa e280 headphone jack. I had the Infinity Kappas I was thinking of getting powered by a Sony Amp with 100W/channel. The radio sounded really good but then my mp3 player the sound was much more muffled and not as clear.So that got me thinking, is the sound clarity from a head unit playing off a USB stick better versus a playing off the headphone analog jack? Does anyone know?I was very disappointed with what I heard from the headphone jack.

Thanks,Dan