We have used both the #3 and #5. I like the #5 for garments it has big teeth.
The #3 is lighter, and not so large, I feel like the #5 is overkill for a sock or overcover. However, I think it comes down to a matter of taste and the fabric of choice, I would not put the larger zipper on really light weight fabric.
When we were researching zippers, DIY Gear Supply, was very competitive and we know we will get what we order in a speedy manner. The coil zippers are reasonably tough, so you really do not need a big old zipper. Plus I think the #3 would be a bit less prone to water seeping in during a storm or heavy mist.
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