I was in a similar situation in a 5 story building. A coffee pot or wiring (not sure which) caught fire on floor 2 or 3, and I was on 5. We could smell the smoke very strongly going down, so it might've been more of a problem with a larger, more ominous fire. I think going down is clearly the best.

One off-topic thought I had was that if it had been a chemical or biological attack on the inside of the building (such as explosion or ventilation system), going up might be a better choice depending on the current floor and the most-affected floor.