I have both right in front of me. Amazon US has them. So, in my ears I hear some very minor brightness, or ESS glare / sheen on the 15i. Everyone hears a little different. It is very minor but I can’t not hear it. I have always been sensitive to this. In addition I believe the CS is slightly warmer and has slightly fuller vocals. In addition I like these better than the Moonriver Ti, DC04 Pro, and the Aful Snowy Night.
I also can’t listen to most metal dome speakers but that is another issue.