This depends entirely on what you want to connect the toslink/aux cables to.
If you are running the surround system off an AVR, then it is likely that the AVR will have better DAC than the PC, so the toslink is better (provided the PC outputs a format that the AVR can receive, but that shouldn't be an issue).
If you are using some other way to power the speakers then it's likely that the different-aux-cable route is your only option, since you'll need to pass an analogue signal to the speakers somehow.
If you happen to have an AVR that has individual RCA or aux inputs per channel, as well as toslink, you can even experiment and see which sounds better. But overall I would probably recommend letting the AVR handle the DAC duties.
You can maybe also run HDMI from the GPU to the AVR... I haven't tried doing it that way but if the PC sees the AVR as a sound device that might work too. Then it would be the same as (or better than) the toslink route as the PC would be passing a digital signal to the AVR.