Answer:
Can't really say as it depends on brand and company of the makes, and what materials were used in the plastic(as not all plastic consists of the same things as plastic is a combination of many different materials, some toxic and some nontoxic), but my guess would be the paper bag(even though not all paper is the same either), because the ones from albertsons and the old save mart before they took away the bags, were thick as heck the plastics were so thin, if you tried to hold two 1 gallons of milk in there, it would surely rip.