usb, spdif, or usb to spdif converter

If both a source and dac have both usb and spdif connections, generally speaking which is a better connection, and is it ever advantageous to use a usb to spdif converter over straight usb or coax? It was my understanding that these devices were developed for legacy dacs with no usb input. usb wins over coax in my system (computer based) but I am considering purchasing a converter if that will improve things. Thank you.

Many thanks and hope you all had a wonderful Thanksgiving.  I am running a windows 7 desktop computer (a 12 year old gateway) usb output to an idsd micro dac for both music and blu ray movies.  I would rather use a stand alone player.  However, when I compared the computer sound (usb to the usb input of the dac) to the player's sound (coax to the same dac - using a newer sony blu ray player) the computer's sound was MUCH better in every way.  Any thoughts on getting equivalent sound via coax from a player would be appreciated.   

I've got a music fidelity sitting here doing nothing if you want a fabulous deal on a USB SPDIF converter.  This is the 24/96 version.

Which is better probably depends upon your hardware.  The SPDIF is more generic, I think, so it might all depend on how your DAC handles USB.  DACs convert USB to I2S internally, so that if your DAC does this well, no need for external USB to I2S device like the Singxer.  

Ultimately, a sound test is best.  However, for example, if the USB board in the DAC gets its 5V internally, as opposed to through the USB cable, it is likely the best way to go.  This is something to consider when buying a new DAC.

The sony blu ray players are an okay transport for digital coax s/pdif, but they are certainly not the best.  To be honest, most of the bluray players out there are not the best S/PDIF transport.  If you want a really good player/transport for both movies and music, look for a used Oppo 103 or 203 player and then upgrade it with a linear power supply from

Also, your coax digital cable will affect the sound quality you get as well.  Make sure you have a good one.

Otherwise, you can run with a computer if you want.

One more thing.  If you are running the ifidac using coax spdif, you may want to look at an outboard USB linear power supply, like the LKS:

Unless you charge the Ifi DAC separately and then unplug the usb cable before turning it on with coax input.