From what I've read, all of the types have different tradeoffs. The thinnest plastic ones technically have the smallest carbon footprint because there's less weight to ship around, the materials are easier to acquire, etc.

My personal reasoning is that there are lots of other (more significant) ways to combat climate change, but there's no other way to stop whales from choking to death on plastic. Then again, I could just be more emotionally affected by the tangible images of plastic pollution than by the more abstract notion of carbon footprint.