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.