picking the right Vgs th based on VOH and VOL
It sounds like from the description that you provided that you are intending to operate the MOSFET as a saturated switch. If this is the case and if the selection is for an N-Channel device then it is pretty easy to see that the MCU pin will have no trouble turning OFF the MOSFET. So the more important consideration becomes the available gate drive voltage to get the device turned on hard enough to satisfy your load switching requirements. You want to get the RDSon low enough that the part is not dissipating too much power at the load current that you need it to switch. You also want the low RDSon so the device will pull the load as close to GND as possible so the load gets the benefit to as much voltage across it as possible.