A closure that actually closes
A zip is the only closure here that genuinely contains contents, which is why it is specified wherever items would otherwise work loose. It also gives the pouch structure. The trade-offs are cost and feel: zip quality is the detail most often approved from a photograph and then changed, and a zip needs seam allowance all round plus a straight run for the tape, which constrains the shape the pouch can take.
How the zip performs in use
A zip contains contents properly and adds structure, which is why it is the closure specified wherever small items would otherwise escape. It is also the component most exposed to wear: the tape, the slider and the pull are three separate quality decisions, and a cheap zip undermines an otherwise good pouch faster than any other single part.
Run the zip repeatedly on a packed sample. Smoothness under load, and whether the slider wanders off the tape at the ends, are not visible in a photograph.
