USB or S/PDIF for Simaudio 100D

Wondering whether to use USB cable or S/PDIF converter, to connect new iMac to Simaudio 100D DAC, then to Onkyo A-9555 and ERA 4 speakers, maybe ERA 8 sub. I already have a few good digital RCA cables, so cost of converter or USB cable will be close. Thanks.
All the music on my computer is from redbook CD
I owned the 100d. The USB input is not that great. Even the manual recommends against it. Better to go SPDif or toslink.
Thanks Meiwan. Was there a converter you found to be a good match with the 100D?
I never tried one. I used glass toslink from my iMac - mini rca to toslink. Lifatec silflex works great for about $70. Less messy than USB to converter to DAC.