Digital cables: 75 or 110 ohms?

They should be. That is how the S/PDIF (coax) and AES/EBU interfaces are specified. The balanced cables are more RF-noise-immune and carry higher signal levels. There are other differences in the data protocol but, overall, they are similar. I generally use the balanced if possible.

The SP/DIF spec is 75 ohms, single-ended signalling. The AES/EBU spec is 100ohms balanced or differential signalling.

AES/EBU spec is 110 ohms, not 100.