Is 14 mm or 18 mm better?

Neither. The right one is the size your bong takes. A bowl that doesn't match won't seal.

Can I put an 18 mm bowl on a 14 mm bong?

Not on its own. It's too wide to go in. You'd need an adapter, and we don't sell one.

What do 14/20 and 19/22 mean?

That's how lab glass is labeled: the width, then the length of the frosted part, in millimeters. So 14/20 is a 14 mm joint with a 20 mm frosted section. On bongs the same joints are just called 14 mm and 18 mm.

14 mm or 18 mm bowls, and which fits your bong

Your bowl has to be the same size as your bong's joint. A 14 mm bowl won't fit an 18 mm joint, and an 18 won't fit a 14.

Size	Actual width	Bowls we have	Bongs and bubblers that take it
14 mm	14.5 mm	31	53
18 mm	18.8 mm	8	1

Not sure which you have?

Use a US dime. It drops into an 18 mm female joint and sits on top of a 14 mm one. More ways to measure

14 mm bowls

53 of our bongs and bubblers take this size, and we have 31 bowls for it, from $8.49. The cheapest four:

2" 14mm Maria Color Lip Flower Bowl $8.4914 mm male
2" 14mm Clear Bubble Flower Bowl $8.9914 mm
2" 14mm Color Heart Flower Bowl $9.4914 mm male
2" 14mm Clear Flower Bowl $9.4914 mm

See every 14 mm bowl, sorted by the joint it fits.

18 mm bowls

2.5" 18mm U&M Color Illusion Flower Bowl - Made in USA $12.1518 mm
2" Black Color Lip Flower Bowl from $12.9914 or 18 mm male
2" 18mm Fume w/ Color Glass Flower Bowl $13.9918 mm male
2" Full Color Flower Bowl from $14.9914 or 18 mm male
2" 18mm Color Swirl Can Flower Bowl $16.9918 mm
2" 18mm Color Worked Joint Flower Bowl $16.9918 mm male
2.5" 18mm U&M Color Bucket Flower Bowl - Made in USA $19.9918 mm male
2" Element Color and Fume Flower Bowl - Made in USA $20.9914 or 18 mm male

Why they don't swap

A bowl that's too small drops in too deep and wobbles, and one that's too big stops at the rim and won't seal. Get the size right first, then male or female.

What the numbers mean

The real widths are 14.5 mm and 18.8 mm, measured across the widest end of the male part. Bong shops round them to 14 and 18, and some call 18 mm "19 mm". The sizes come from lab glassware, which uses the same joints.

If your downstem comes out

Then the bowl goes into the downstem, not the bong. Downstems are named with two numbers, like 18/14: the first is the end that goes into the bong, and the second is the end that takes the bowl. So an 18/14 downstem takes a 14 mm bowl, even though the bong's joint is 18 mm.

Questions

Is 14 mm or 18 mm better?: Neither. The right one is the size your bong takes. A bowl that doesn't match won't seal.
Can I put an 18 mm bowl on a 14 mm bong?: Not on its own. It's too wide to go in. You'd need an adapter, and we don't sell one.
What do 14/20 and 19/22 mean?: That's how lab glass is labeled: the width, then the length of the frosted part, in millimeters. So 14/20 is a 14 mm joint with a 20 mm frosted section. On bongs the same joints are just called 14 mm and 18 mm.

Sources

The real joint widths: the ISO 383 list of glass joint sizes, and ground glass joints for how they narrow.
The shop names for the sizes, including 19 mm: Smoke Cartel's joint guide.
Downstem names: Thick Ass Glass's downstem size guide.