The ramp tour is great. The stop at the runway cross takes about 10 - 15 min (only).
The tour starts at the far end of the observation deck (Terminal B). From there you may also make some great shots of departing a/c.
The best time for the tour is about at lunch. You can see the 747s departing and the view is the best. You may ask the Bus driver to stay longer at the runway cross and wait for the next bus. After youve seen the runway cross they take you to the home of the firefighters.