dac is much cleaner cos it operated on separate circuits and it has clock processing/upsampling with separate dac staging caps and opamps...
not to mention is it has own separate toridal wound transformer to supply it's with power...
soundcard is still drawing power from computer psu and most prone to processor fans eletrical disturbances and surrounding pc parts noisy and heat generated/moving parts.
but of cos in the end it's up to u to decide