Using a 3.3V (max. 8 mA) output pin off an Intel Galileo development board, I need to turn on and off a 18V DC motor. Is it better to use a JFET or MOSFET transistor?
I doubt that a JFET will be able to adequately control a motor without significant power losses due to it's likely high on resistance. MOSFETs, on the other hand, are extensively used for controlling DC motors and power-hungry loads.
The best JFETs I've seen might just about get as low as a few ohms whereas I've seen MOSFETs that are that value divided by 1000. Also, JFETs tend not to be rated at voltages in excess of 30V.
A JFET, as a generalism, is not suited for motor power control.