Stack 'em high
This is one of the enduring points of debate about home brewing. Old School says that sure, glass is heavy and breakable, but it's also impermeable, easy to clean, doesn't scratch and lasts until it breaks—how can this even be argued when plastic breathes, is tough to clean effectively, easy to scratch and wears out?
It turns out that the Old School might need some new data.
We're talking about modern plastic, polyethylene terephthalate (PET). It's vastly advanced compared to the white plastic pails and buckets which were made with High Density Polyethylene (HDPE). Rigid, white, light and flexible they work pretty well, with the caveat that because they're safe for food that they don't last forever: food grade means they don't contain plasticizers (which keep the HDPE from getting brittle) or UV treatments to prevent UV rays from making them (you guessed it) brittle).
The stuff in PET Carboys is actually a polyester compound and is the same stuff (Dacron) that went into groovy paisley shirts and clip-on ties in the 1970s. In the case of PET Carboys it's rendered amorphous (clear) and is sandwiched with a layer of PVOH (Polyvinyl Alcohol) plastic to block oxygen permeability—more on that in a minute. PET plastic won't absorb odours or stain from beer or wine—it's non-porous and hydrophobic—so it won't carry over colours or flavours from one batch to the next.
Let's look at the concerns:
1) Plastic breathes
PET plastic has a rate of oxygen permeability that's only slightly lower than glass. It's so low that tests have shown that there is more oxygen exchange from the interface between the bung and the neck of a glass carboy than there is through the wall of a PET carboy!
2) Plastic is tough to clean without scratching
PET will scratch, so using a rough-bristled brush is out. However, unlike other plastics, PET is immune to even very harsh cleaning solutions. If you've got a gooped-up carboy just soak it overnight in PBW or similar and it will drop clean like a magic trick.
If it's light soil, just put a couple of cups of your cleaning solution in the carboy, toss in a soft cloth and roll the carboy around on the counter or floor. Because the inside of the carboy is so low-friction, the weight of a wet cloth is enough to scrub it completely clean. Dump it out, retrieve the cloth, rinse and you're good to go.
3) Plastic wears out
PET is non-reactive to UV light and remains pliable even in the long term. The first PET Carboy carboys to come off the line are still in use today, after nearly 20 years of service!
As for the advantages of PET Carboy:
The combined weight of 19 litres of beer or 23 litres of wine and the glass carboy to contain it adds up fast, to 25-30 kilos. PET Carboys only weigh a couple of hundred grams, lightening the burden—especially nice if you're lifting it up for racking or into an awkward space, or if your back just isn't quite what it used to be.
So many busted carboys . . . art installation by Robert Smithson
The biggest factor in favour of PET Carboy is that they're essentially unbreakable under normal use. If you bump a glass carboy against a counter or another carboy, or if you lift it by the neck and it has a weak spot, or heaven forbid you actually drop it on the ground, it's going to shatter.
When a glass carboy does break, not only do you lose whatever delicious beverage it was holding, you're exposed to a dangerous shower of glass. Every homebrew shop owner knows an awful story of emergency room trips, stitches and in some cases really appalling injuries from broken carboys—for that alone, PET Carboys are a great carboy.
Getting dry hops in and out of a glass carboy that has a 6.5 bung opening is truly painful. The neck of the PET Carboy fits a alarger bung and is a lot easier to use to add oak chips, hops, spices, or anything that needs to go into—and eventually come out of—your homebrew.
If you're interested in setting up a really convenient system for racking from one carboy to another it's easy to set up the PET Carboy with a valve and spigot, along with an o-ringed bung that will let you transfer with little to zero oxygen transfer! Glass carboys with spigots are out there, but they're expensive and rare.
Finally, we like PET Carboy because of how it is when it's empty: it's so light you can store it nearly anywhere it fits, on top of a closet space, in the attic, etc. They're so light it's a breeze to put them somewhere high up, and because they won't break you don't have to take a lot of care when you stack them up.
The biggest everyday reason to use PET Carboys is their light weight and ease of cleaning. Long-term, their sturdiness and resistance to shattering gives them a huge advantage over glass, while delivering great results.