Preferred Audio Out from iMac, 3.5mm or USB?

What is the preferred 2CH audio out selection from an iMac to my preamp, USB or the 3.5mm headphone jack? Either output will run through a separate DAC before the preamp. Mainly listening to iTunes or Pandora.

If using the 3.5 mm output, a tosslink to mini-plug cable will be used to connect to the DAC. If using the USB output, a USB to Mini cable will connect to the DAC.

Thanks for the reply. The preamp is a Forte Model Forty Four, amp is a Magnum Audio MF330 and speakers are Dali Mentor 2. For now, iTunes content is strictly iPod library and more often iTunes Radio in addition to Pandora.

I'm using Beresford TC7510 DACs with a couple of other systems and would use a TC7510 with the above if there was no audio advantage to use the USB output from the iMac. If USB provides an advantage, I'll need a different DAC, probably the Beresford TC7520.