No on can tell for sure. It depends on lot of things:
- quality of DAC's USB input
- quality of DAC's S/PDIF input
- quality of S/PDIF coax
(if we neglect the importance of USB cable since it will be present in both combos, so let's say there's no comparative difference, though it is important as well per se)
In case your DAC has a decent S/PDIF input quality, supposing its USB input quality is likely not brilliant, and with a careful selection of a digital coax, benefit is realistic - Eitr performs more than one good thing, there are both galvanic isolation and reclock implemented inside, aside of decent USB input.
I know that with Micromega MyDac Eitr definitely improved the sound, but the result is also sensitive to S/PDIF cable used - cable type, connectors used and its length matter.