So, ‘when’? It’s a matter of preference as #JDRoberts shows above.
The Alexa Momentary version handles the ‘Off’ since it is immediately turned Off so that it can be used to trigger something again later in SmartThings or Alexa.
On the other hand, the Alexa ‘Switch’ version will remain ‘On’ until either SmartThings or Alexa turns it ‘Off’ so that it can be used again later in SmartThings or Alexa.
So the Alexa Momentary version is the way to go in most situations. However, a use case for the Alexa Switch version is to have Alexa or SmartThings turn it ‘Off’ as that is a confirmation that the Alexa Routine was triggered or the SmartThings Automation, SmartApp, or the STHM was triggered, and it confirms that SmartThings and the Alexa connected service integration are communicating with each other probably between themselves.
Notice that I included the STHM in the Alexa Switch version. I included it since Alexa could have a sensor/system that can’t be integrated with SmartThings, so the Alexa Switch version could be the Open/Close sensor monitored by the STHM for an Alexa sensor/system. I don’t have a use case scenario of any sensor/system that’s only available through Alexa, but it is possible since these Alexa virtual devices work both ways.