Long runs - USB or RCA???

I'm considering an "external HD > laptop (PC - iTunes) > USB DAC" setup in my office. My audio rack is 15' away from my listening position where the laptop/exHD will be. My question is - do I position the DAC near the listening position and run 15' analog RCA cables to my integrated amp in the rack or do I site the DAC in the audio rack and run a 15' USB cable to the DAC? I know that with certain RCAs, I'll be susceptible to RFI; will a long-run USB cable potentially have the same problem? Any recommendations of either flavor cable to avoid the RFI? Thanks.