Propylene glycol is considered by the FDA as GRAS (generally regarded as safe) and can be used in foodstuffs etc. Although drinking large amounts is not to be recommended! Ethylene glycol is poisonous to humans and animals. Propylene glycol is converted to lactic acid in the body (what your muscles produce when you perform rigorous exercise) whereas ethylene glycol is converted to oxalic acid, which is toxic and causes among other things convulsions, kidney failure and death. More information available at :
[URL="http://www.atsdr.cdc.gov/tfacts96.html"]

50% propylene glycol with water freezes at -26degrees F.

I bought a gallon today at a local hardware store for $4.50

Clearly propylene glycol is preferable for this use.