I believe people use the unsealed for bottom feeder mods. The unsealed have a hole through the positive pin in the connector.
Unsealed is also for use with 'auto' activated mods, where there is a pressure switch inside the unit, such as what most of the store-bought cig-alikes have.
Otherwise, unsealed would likely be a better choice.
Some just fill the hole with epoxy or solder if they have an unsealed, but don't need the thru-hole.