full theory from here:
http://www.learn.eversmoke.com/pg-vs-vg-based-e-liquid.html
Vegetable glycerin is a thick, heavy, and sweet-tasting liquid. It gives your smokeless cigarette a very pleasant sweet flavor, and creates large amounts of vapor. It also creates an incredibly smooth
throat hit that many users highly appreciate. Vegetable glycerin is derived from, you guessed it, vegetables. Therefore it is considered completely natural and safe. VG is also used as a sweetener and in many cosmetic products.
Vegetable glycerin imparts a slightly sweet flavor –however it does not contain sugar, and is metabolized by the body in a different manner than sugar.
Propylene glycol, by comparison, is very thin, but even sweeter than VG. PG is known to produce a great
throat hit, and that is why some people favor it in their smokeless cigarettes.