An adjustable pouch will allow you to adjust your pouch as required to make it a little bit smaller or bigger should you loose or gain weight and need to move a size up or down.
You will still need to give us a "middle" size so it's best to measure yourself and let us know what size you currently need even if you do decide to go for adjustability.
We use snaps on our adjustable pouches. My own pouch is adjustable and I can confirm that they do not snap open during use.
The pictures below are of my own wrap adaptation pouch sling and show the snaps. I used red snaps on mine but you can request a colour of your choice.