I use a chiller. When the humidity is high, fans do not work as well at providing evaporative cooling. And let's face it, the humidity has been high for a couple of weeks now here in Maine. for over 11 years, I would point every fan I could get my hands on toward the tank and turn off the halides on the warm days. There's a lot to be said for overall system volume too though...
I used fans for a long time, and I bit the bullet and ordered a chiller last year. I haven't lost a piece of livestock to temperature spikes ever since. My clams and SPS seemed to be especially susceptable to high temperatures, and it broke my heart to lose a clam to high temps.
I also use a large fan blowing across the top of my tank, but the chiller takes care of business on those unbearably humid days.
-Rob