The more expensive one is a PWM (pulse width-modulation) fan. It has 4 pins for connection and is meant for the mobo to adjust the speed using the 4th pin (PWM control). It CAN work on a 3 pin fan header, but you will have paid for the feature and neglected it.
The less expensive one is a 3 pin, voltage in/ground/speed sense fan, without the PWM control pin. It is cheaper to make.
Look at your motherboard and see what kind of headers for fans you have. Typically most mobos have at LEAST 1 4-pin PWM capable header for the CPU fan, and some mobos have 1 more 4-pin for the rear case fan. The remaining headers are usually 3-pins. So check your particular board and see what you have. If you are only planning on using these for case cooling and/or you have a fan controller (without PWM, check the pin count), buy the cheaper 3-pin variety.