Both are gas assist. Never had the need for but if you're worried about air coming through the burner, then just wrap the venturi holes with aluminum foil. To avoid unwanted air leaks, it is very important that your lid, bowl and along with the stacker must be as perfectly round as possible.
I use a stacker for both low and high heat cookimg. This is a 14" stacker while cooking up some 2 racks of Vortex wings.
This photo is from couple days ago while while hanging 7 racks of ribs.