I know that there has been tremendous improvements in USB connections in the last couple of years, but strictly my opionion -- I avoid USB connections so the mobo just has to have SPDIF coax and optical -- unless you have chosen the DAC of your dreams already. USB means drivers for Windows, potential compatibility issues, adjusting buffers maybe etc. Not that there aren't very good DACs that can sound very good ... there are. I just like to avoid complications. So for me, non-USB digital outputs to an external DAC, and just listen to the music.
Internal Sound cards -- probably need drivers again so I'm also not a fan. Although there are high end "interfaces" from Lynx, Sternberg, FocusRite etc. which might suit better for people who are using convolution ,setting up active filters, or need multi-channel to avoid HDMI playback for audio. I just don't call these products sound cards anymore, but I guess they can be considered such.
There are pros and cons for both coax and toslink; some dacs don't have high-rez sampling support over and above 96kHz or 192kHz (Your Audionet only supports digital inputs for USB and Toslink limited to 96kH). If this is important to you, this could be a consideration. CHoice might have to do with distance, and unfortunately Astromo is right, often you can get a mobo with one digital out but not both. There are choices with min-ATX boards though, the tiny itx things can be limited.
Regardless, there are a lot of high end dacs that seem very pro USB, I just find they can be more problematic when needing native drivers, updating firmware etc. So I look for motherboards with all the options when possible.