Whats the difference? If you order a bong and it gives you the option of 14.5 or 18.8 which is better? Does 18.8 have less drag or something?
18.8 only makes a difference with carbon filters imo. I prefer 18.8 just because they seem easier to pull out and put back in for some reason.
Smaller joint size on a smaller piece ? Airflow also depends on your slide and screen.. Or ice pinch slide or something similar..
Part of it is preference, other part is design. 14mm is small and allows the slide or A/C to sit closer to the center of the pipe hence why 14mm downstems are called low profile. Where as 18mm usually requires space for the downstem to expand back to 18mm so it sticks out more. When using a ash catcher or profile this could mean the difference between a nice setup and one that fell over and broke. As for better draw and other things, no one has done the mathamatics to say which is better. 18 gives more air, but 14 makes faster air which means a faster hit.
it really doesn't make a difference. I'd go with whichever has better accessories. Around here it seems to be easier to find stuff in the 18 size so most of the glass I buy is 18. Back in VT it was the opposite though, who knows