GM's hardtops first appeared on 1949 model year Oldsmobiles, Buicks and Cadillacs, then on 1950 Chevrolets and Pontiacs. As noted in the link above, I am very pleased with the design GM stylists used for those tops. But Oldsmobiles, Buicks and Cadillacs got a complete restyling for 1950, and that created an opportunity to create a different hardtop look. What the stylists did was design a lighter appearance by reducing the size of the C-pillar, as can be seen below.
As for Chevrolet and Pontiac, their next redesign came for 1953 models. Here a thinner but still substantial C-pillar was used. These pillars were given a backwards slant that nowadays seems to be associated with BMW styling. One human-factors consequence was improved viewing by back seat passengers, as was a result of the 1950s restyle of GM's senior lines.
Nevertheless, to my taste, the first-generation GM hardtops looked better than their successors.