Hi T.C.
Welcome to the Wide World of Screenprint!
Short answer is PVC-free Plastisol, because of the additives used in Discharge inks.
But there's "more to this than meets the eye", and the ink performance and application differences make for an unequal comparison.
"Safer" is a tricky word; a good place to start in understanding what a company will rate as "safe" is a Federal Document System that is mandatory for all industrial manufacturers and the products they make for you called a
Material Safety Data Sheet;
http://en.wikipedia.org/wiki/Material_safety_data_sheet
Note that OSHA, your local Fire Dept., and your local Dept. of Environmental Quality, will all often define "Safe" differently.
Be wary of product manufacturers advertising claims, as they will not always point out that PVC-Free Plastisols have simply replaced the type of "plastic" compound being used with another.
Always look to any other certification they can provide.
Here's an example of a water clean-up hybrid acrylic ink that is considered relatively "safe":
You're doing the best thing.....
......keep asking questions until the clutter becomes understandable.
Happy trails!
PVC Free plastisol vs. water based discharge 
Re: PVC Free plastisol vs. water based discharge 
