What’s the difference? Is one better than the other? Why? Thanks in advance.
My take is that these names are often used interchangeably for the same thing.
The names are used interchangeably,
though Dry Sack is also a brand of sherry.
But the names are used to refer to a very wide range of bags and sacks, from plastic trash bags twisted or taped shut, to flexible plastic bags with roll top closures, to really expensive, serious, totally dry bags like Watershed, with its rubber zip closure.
So if someone says they have dry bags to go along on your trip, ask what they mean. And ask them to bring some Dry Sack too.