I would invest in a JCAT USB card ($495), though it’s probably more than you want to spend. Alternative, the Matrix Element H USB card ($329) or the SOTM tX-USBexp USB 3.0 card ($350) might be options. That being said, it’s hard to say whether the motherboard USB output would be better or worse than the motherboard optical output. You would just have to test to see.
If you are using an AMD processor, then look into a Pink Faun S/PDIF card ($360). That will likely be better than a USB card, since you are not looking for DSD.
This entire post needs some background info, First of all, all things equal, and for good technical reasons, USB > SPDIF and within SPDIF Electrical > Optical. So an SPDIF card would be pretty much the last choice.
So USB has the most potential, mostyl because it is aysnchronous and allows the DAC to reclock everything. The DAC likely has a better clock than the transport, and there is no long cable to mess it up, so less jitter.
Now, if you are using the solution with the best potential, which is USB, you have the further option of isolating either the sending side or the receiving side. Either works. I dont know enough about he internal design of either to be sure how well isolated one of the other is. Assume the sending device is poorly isolated - all PCs, Macs and laptops are poorly isolated (unless you buy one with a custom card, like an Allo streamer).
Optical (toslink, SPDIF) has one advantage: in a high EMI environment it is immune. That’s why its on TVs.
That said, one of this first posts hit the nail onthe head: with the equipment you describe the differences are likely academic.