First of all,
to the forum
.
Like several others on this forum, I own both a C6 (my daily driver) and a new CTS (my wife's). As you said, they are very different cars. Not knowing where you live makes AWD vs RWD difficult to answer. If you are like me and live in a warm weather state where it seldom snows, don't bother with AWD. You simply don't need it. If you like the sports car type of handling, then you probably want the FE3 suspension (only available with the Y43 Performance/Summer Tire package). The summer tires will not be the greatest tires for rain or winter conditions, so those factors might make you consider the FE2 suspension, or have a set of winter wheels/tires to swap to for the winter.