They are both beautiful cars.
Exterior wise, I like the permanent headlights on the SS as opposed to the "hideaway" ones on the WS6. When the hideaway lights break, it looks terrible when you see one up and one down. I also think that the WS6 is a little more "in your face" looking than the SS, which is more of a sleeper. (The Ram Air hood on the WS6 is more prominent than the hood scoop on the SS.) A Firehawk, however, does away with some of the “in your face” attitude and has more power to boot. How can you dislike that?
Interior wise, there is no comparison. (Yes, I am Pontiac biased.) On the WS6, the gauges are all bordered, the stereo looks and sounds better, and everything is red-lit at night. This may seem stupid to some of you, but, the steering wheel on the SS is ugly. I looks like it belongs in an Impala.