PET is polyethylene terephthalate, which is considered safer as it does not leach BPA's into the liquid. These bottles are clear as opposed to opaque, and are harder/firmer and thus more difficult to squeeze. And I think, as Mann mentioned, they do not allow airing/steeping as readily as LDPE (Low-density polyethylene) bottles. These bottles are opaque, easy to squeeze, allow for better steeping, but also leach chemicals such as BPA's into your liquid. So, be aware of the difference so you can make an informed choice.
This is some info, from awhile ago, I updated for this post.
From what I have seen, LDPE #4 is supposed to be safe, but I am not sure if the nicotine has a corrosive effect or not. I like PET bottles for their rigidity, sleekness, transparency and safety; but even with all our efforts, there is no way to escape the multitudes of all types of lower-grade plastics in our everyday life. All types of plastic are everywhere.
Just an FYI, a
2008 study showed that even PET bottles leach into water, but at levels "well below the US Environmental Protection Agency (USEPA) maximum contaminant level (MCL)," i.e., "safe" levels. What does all that mean? According to that study, "climates where exposure to extreme conditions can promote antimony release from PET plastics;" so even good plastic leaches even if at "safe" levels, but extended time in hot climates exacerbates, if not, promotes, that leaching. For me, if I have bottles that are not getting vaped up, and I have more than enough to warrant a transfer, they're going into PET for medium term, or glass for long term. I also keep bottles in a cool, dark place, period. If traveling, they never stay in a hot car, especially if in any kind of plastic, PET or LDPE.
I should also go on record as saying that I had no problem vaping an over-a-year-old BWB RY4 that was in LDPE (I have no idea how it was stored, but it remained in LDPE for almost a year and a half). Had it leached? Dunno. Was it good. Yes. Will I make that a habit? No.
The only one *rule* I ever consider a given is that there is an exception to every rule (barring any moral laws I prescribe to).
Edit***The reason Ahlsuion uses PET as opposed to LDPE (now), as stated on their site, is because Wlad did testing (GC/MS) on liquids stored in LDPE and found evidence of leaching. How much? I don't know. It was enough for him to remove LDPE from his operation (he used to sell in LDPE) and replaced it with PET. But, I don't know the conditions that the bottles were stored in, so that is not to say that LDPE is bad, but just know that it
may not be the best.
). If I just get a liquid that I have never tried, I'll steep it in a 3 mL LDPE bottle for the fastest steep short of "doing" something to it, e.g., hot water, ultrasonic, sunbath, etc.
If I have a juice I don't care for I just add to one i like or try to doctor it up to vape it 
