For a given time/speed/distance, I beleive that when crunched for time, the fixed is a better workout. The effort is more constant, hills are harder, and the continual moving of the legs gives a bit more calorie burn.

I've ridden the same loops both fixed and geared while at the same time looking at the average speed for each loop. If the average is the same, I have always felt more spent if the ride is on the fixed.

So, given a short amount of time to ride, I will reach for the fixed.