In practice asynchronous USB is definitely superior. On well designed units the jitter performance on S/PDIF and USB will identical but for a lot of DACs jitter is often worse on the S/PDIF input;
USB v S/PDIF. USB has more options such as high bit rates, DSD > 64 and controlling the volume from the streamer. Some DACs might just give one or two to extra options but, that is one or two more than S/PDIF.
Thing's like like low noise and/or galvanically isolated USB are often discussed but most users do not have problems with noise or earth loops. I could not find the link but I sure
@Amirm has tested various low noise USB PSUs and isolators and found they make no difference.
Buy a DAC and a RPi, and load your software of choice. If you do have problems with noise or earth loops - and most users do not - try a different "off the shelf" PSU or a different outlet on the mains before you spend money on linear PSUs and galvanically isolated USB interfaces.