I use 15 ga for nailing base and casing, on the casing nail with the 15 g to the studs and 18g to the door or window jamb.
T&G ceilings, azek, door frames, pretty much anything.
I have the Hitachi nailer, it works fine, but its loud ( I have ear plugs 85% of the time anyway). The onething that I love about it is the air blower to dust off yourself, other tools , or the cabinets with dust in them, I think all trim guns sould have this feature