The CTS is better on execution. But the Genesis has excellent interior room, it is equal to the s class but both the CTS and STS have low interior space for their exterior dimensions. The pontiac G8 was also in the size comparison and it had more interior room than the 7 series. Having the Cadillacs on Zeta should resolve any space issues that the Sigma platform has.
Here is the link to the article:
http://www.automobilemag.com/reviews...sis/index.html