Most of these type of fans have a fair tolerance so If it were mine I would run at 28v.
All resistors basically have the same effect, they limit current, which in turn causes a volt drop or potential to appear across the resistance. Saying a 'current limiting resistor' and 'voltage reducing resistor' is like expressing the same thing in a different way.
Actually using a resistor to drop voltage is very wastfull as you can see by the wattage (heat) dissipation of the resistor.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.