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Going USB to SPDIF should give you better performance because it the USB part is buffered and thus, in theory, jitter free. You also have the advantage of being able to run a long USB line if needed from the Mac to the receiver.
The HagUSB is an excellent USB to SPDIF converter if you don't need any extras. Comes in kit or ready built form.
Couldn't you just use an Airport Express to wirelessly send music from your Mac to the AE and then Toslink from AE to the Denon? I do this with my iMac to a TriVista and the quality of sound is indistinguishable from playing the CD in the TriVista. I don't know if you can use an AE with the Mac Mini, but if you can, the AE will get you to where you want to be for $129 total.
The digital input on my Denon is an RCA, so I need something that will convert either a USB or Toslink to RCA. I decided to try the following: going out of a new Mac Mini via its Toslink/miniplug using an optical cable that is terminated with Toslink on one end and miniplug on the other (Van den Hul OPTOCOUPLER Mk II) and into a Monarchy Audio DIP Classic. Then out of the DIP via a digital RCA and into my Denon. This combo might be interesting to compare to going out of the Mac Mini via USB into a USB/Coax converter box and into the Denon.