The advantage once zip is that it is airtight, so no worries whether to keep fresh or dried products that should not get wet. I used to put my garden to plant seeds and it is really great!
The plastic of the bag in itself is not so fragile. It has good resistance with heavy things inside, which is a real plus for the price.
They are worth, these small bags and by that 500 are used, they are people for a long time!