Rega Dac USB vs Toslink

Get a glass toslink cable and compare IMO. I bought a Sonicwave glass cable from Amazon for about $25 shipped. It sounds so much better than any fiber cable (read plastic) that I've tried. Some of those plastic cables weren't cheap.

Toslink has an advantage that it eliminates electrical noise.

The Rega DAC reclocks, thereby theoretically minimizing/eliminating jitter issues. USB and toslink are reportedly both higher jitter interfaces.

Regardless of if reclocking negates differences or not, the only way to tell what sounds best is to try all reasonable options. The glass cable I spoke about is $25. If USB sounds better, I'm sure you could find another use for it.

The Rega DAC's USB input is limited to 48khz, I believe. Toslink will go higher, so if you have high-res stuff you may be forced to use toslink or get a USB to coax converter.