If it has to be one of the two, get the Pontiac. That's still far from the best choice. My aunt had a Grand Prix GT (3800 V6) and the car gave her nothing but trouble. In less than four years the transmission failed, the rear main seal blew, and the engine threw a rod. (All separate incidents.) There were also persistant issues with the traction control and the ABS.
ZV
EDIT: My aunt still ended up trading her old Grand Prix GT in on a new Grand Prix GT though. She just likes sporty cars. Before the Grand Prix she had a Mustang GT ('94 I think, first year after notfred's style of 'Stang) and before the Mustang she had a Camaro. Then she got tired of RWD in the winter and bought the Pontiac.