Most non-PG is VG. VG is easy to get and gives more fog. However, VG is known now to leave far more deposit in the atomizer (actually, PG leaves almost no deposit). This might not be a big deal because a lot of the deposit comes from the flavorings (juices vary widely in this regard, a ballpark figure would be that VG contributes about as much as the average from flavorings, meaning PG is decidedly better for getting less deposit buildup). One alternative to VG is PEG-400 and the experimenters will be testing this out soon to see how it fares deposit-wise.
PG is less thick than VG and uses a bit less power to make vapor. A minority of people don't tolerate PG well (check out some other
threads if concerned about that).
A few juices contain both PG and VG.