Because you cook with lots of water in a stockpot, it doesn't have to be heavy. You won't be browning or searing in the pot, so no need for a heavy bottom. If you might want to use the pot for making big batches of something thick, like chili or stew, then the thicker one would be better. But just for stock-- I would go for the less expensive one.
I just haven't been the same
since that house fell on my sister.