S/PDIF sounds more analog than AES/EBU

You should audition each cable separately.If both are connected and you are flipping a switch,then one is contributing to the other-both are "in the circuit".

Chris - As far as I know AES/EBU is the same thing as S/PDIF. It is used in studio equipment while S/PDIF is for the rest of us. Electrical levels are slightly different and it is balanced but the protocol is the same. My Benchmark DAC1 receives both with the same chip. Here is quote from WIKIPEDIA:

"the low-level protocol for data transmission in AES/EBU and S/PDIF is largely identical, and the following discussion applies for S/PDIF as well unless otherwise noted." There is also fiber-optic version of the same protocol know better from its Toshiba name "Toslink"

I don't know what could be different in your system - maybe this difference in electrical levels. Are both Nirvana cables of similar quality?

I've done some comparisons IMS with AES/EBU and S/PDIF (RCA) cables and find that S/PDIF generally provided a deeper soundstage with a smoother and more relaxed presentation. The AES/EBU were more dynamic and upfront with a wider stage. Preferences depended on the individual cable brands as well as the particular recording I was listening to. Mostly though, I've settled on using S/PDIF at present (though my Tara 'One' has a BNC on the source end).

Tpsonic:
Could you explain in more detail why having both types of cables connected at the same time would affect each other, since one is selecting and de-selecting the inputs. All my comparisons were done with both cables connected and toggling between inputs.

While it is okay to have both cables feeding the DAC, one should never have both cables feeding an amp while using the toggle switch on the amp to flip from one feed to the other.

This can be a disaster as most amps do not shut down one feed as they source to the other.

While making a comparison,it is easy to toggle between two cables to make a decision.But, if both cables are in the system,they will affect one another-as they both will present a load to the transport and/or dac.
Indepently evaluate each cable,one at a time.Your results may not change,as I prefer the S/pdiff myself.More from a dialectric position and sonics.