Normally I know clearly my answer to this question.
First choice: "Neither, I don't need a bag" or "I have this bag with me."
Second choice: "Paper, please. Can you fit it all in one?"
And usually there is no third choice. I try to only accept plastic at stores that offer no alternative when I forget my own bag. I know that plastic bags have become an environmental hazard, a littering eyesore, and a poor replacement for traditional containers and handicrafts.
But today I learned there is a time when paper will simply not work: RAIN.
I asked the grocery to put all my things in one paper bag, leaving the laundry detergent out for me to carry by the handle.
I made it approximately 10 steps from my car before the paper handle broke. I then made the crucial error of allowing the bag to touch the ground - it immediately disintegrated.
I had to make a mad dash another 30 feet to my place, run inside to find a non-paper bag and run back out to reorganize my groceries off the sidewalk. By the time I got back inside, anything that was in a cardboard container was soaked.
So I ended up eating a large dinner and throwing away quite a bit of soggy food that was too compromised to sit on the cupboard shelf.