The 2013 Cadillac XTS & 2014 Chevrolet Impala utilize GM's Epsilon II platform. They both have what is known as a "6 window design" where they have a total of 6 side windows. GM's early 1960's sedans also had this design feature but the shapes were all the same between each division. I've included a 1960 six-window sedan for comparison.
The modern Epsilon twins have unique shapes that blend with the brand's design language. Which do you like the best?