Guys - no difference between USB and Ethernet in terms of bandwidth - at least not that is relevant to this discussion. Very little data is required for audio - remember USB1.1 works just fine.
Hardwired (ie USB, Ethernet, FireWire etc) is always going to be more reliable then WiFi (wireless). Being nothing more then a small radio, WiFi is subject to interference where wire is not.
Technically a USB DAC that skips SPDIF and works in I2S will trump any USB SPDIF implementation. For instance Gordon Rankin's Wavelength series - Brick, Cosecant and Crimson. Also see the Empirical Audio website for more tech.
In my own rigs I have found that the SPDIF cable makes a big difference in the kind of setup Drubin has. I have been very happy with SBs modded by Bolder running Ethernet, and run them using their own audio out (ie using their internal DAC). Cosecant (and I assume Crimson) are better, as is a USB>adapter implementation with a high end DAC. Have had equally good results with a glass Toslink directly from the Mac to the DAC.
Lots of ways to go.