Hands down, the GMC. The ford f150 is not a safe truck. They are safer now days, but the 1999-2002? models are bad. The GMC has better styling, features, and more dependable. Safer too.
My neighbor has a 2005 GMC Sierra 1500 SLT and its very nice. Comfortable for a truck. The Crew cab is really nice, and leaves lots of room in the back. I am not sure if you mean Extended cab which is smaller for a cab or the Crew cab.
Overall, Get the GMC.