IN SUMMARY:
A Variac will give you the best consistent results controlling fan speeds for all fan types, but they are large and heavy, cost about $50 used. The next best thing is a fan dimmer, or speed control meant for use with fans and has a minimum speed adjustment. They cost about $10 Next would be a capacitor type "de-hummer" control and it gives you about 3 usable speeds, if the control & fan are matched. Cost about $10.
VARIAC, POWERSTAT, VARIABLE AUTOTRANSFORMER: $40-100
Variacs, Powerstats (brand names) are actually a variable autotransformer. They are made by wrapping copper wire around a laminated steel donut. They have a knob adjustment that slides a carbon brush over the transformer winding that gives an output voltage from 0 to 140 volts AC. Just like the extra 20 volt boost (140 volt) certain models can be wired for 120 volt input and 0 through 280 volt output. (Like a step-up transformer) Variacs are heavy and usually sit on a table or floor. They are sized for the voltages they will be used for and how many amps they can supply. The smallest ones are about the size of your fist and handle 1 amp loads. The larger ones can be as large as a basketball, weigh 80 pounds and can handle 30 amp loads. I would recommend one with a 3 to 10 amp capacity based on what you can get for the price. New Variacs can cost $200+ but used ones average about $50. Variacs are very efficient.
