Digital XLR vs Digital RCA

In my experience, it depends very much on the DAC that is receiving the signal. In other words, it's system dependent.

My impression has been that, with a modern DAC designed according to standard engineering paradigms (e.g., Benchmark), there will be little or no difference between cables. However, with something based on idiosyncratic technology, it might make a difference.

Is it okay to run both cables at the same time to AB them or should they be run one cable at a time?

This is somewhat of a controversial subject.  I have read some people state that AES/EBU XLR digital interface has design issues and not as good as RCA/BNC.  Others say that AES/EBU is far superior.

You can run both cables to the DAC at the same time and just A/B the source input on the DAC.

How you gonna compensate for the 6db diff.?
Any level control you insert will affect the SQ.