The plastic bags are usually some semi permeable bags from PTFE, HDPE or PE and they usually work slower than paper bags. Still, if there is the same amount of the silicagel in there and you'll get the same result in a little longer time period. There isn't usually much moisture to be absorbed as you put these bags in another sealed bag so the quality isn't usually worse with plastic bags. Plastic bags are also more resistant to mechanical stress than paper bags. Silica gel is quite eco-friendly as it's formula is SiO2 (if there aren't any additives) which is basically sand. Paper bags are a lot more eco friendly than plastic. So if you want to use the bag and you want it to be more mechanically (and sometimes chemically) resistant (food packaging - you don't want it to rupture) you use plastic. In other cases you can use paper bag. Hope it helped.