the stem size will be the same with both the 14 and 18mm on the sub, the teflon piece that connects to the glass will be different sized. On the glass though, I'm not sure if it makes a huge difference with airflow, or not. Personally, I like the 14mm because I can set it into my 14/18 reducer and let it warm up and expand inside that piece, solving any issues of sticking/expanding in the joint of the piece. You do have to keep an eye on the reducer if you let it cool it will slip off, and mine gets oily after some use so I clean it regularly. Either way my sub likes to make the entire lower portion, from the teflon down, get covered in an oil film that builds up. It's just part of the experience, having it in the "buddy" adapter. I've been reclaiming that oil, and it's amazing.