A few words about zippers
Zippers can be installed with the zipper coil exposed to the outside (this is the "standard" orientation), or with the smooth fabric "tape" facing out (this is called "reverse" orientation). In the reverse orientation the coil is concealed and just a small gap is visible where the two sides of the zipper meet. The primary benefit of the reverse orientation is the smooth, clean appearance. The reverse orientation also offers added protection from sand and dirt getting into the coil. The reverse orientation is most effective on straight, flat openings, since the gap tends to flare around corners, and can interfere with the smooth operation of the zipper.
YKK's AquaGuard® zipper series adds a shiny polyurethane laminated film to the woven zipper tape on the "reverse" side of the zipper, which they promote as enhancing water resistance. The lamination film is applied by heat to a standard zipper, so both styles are identical except for the lamination. While many people assume AquaGuard® zippers are "waterproof", they are actually "water resistant" — not waterproof/watertight.
Considerations: You should be aware of two characteristics of AquaGuard® zippers. First, the polyurethane coating adds resistance to the sliding of the zipper, sometimes requiring two-hands to open and close the zipper. This is an especially important consideration for bicycle handlebar bags, where one-hand operation is essential. Second, the polyurethane coating is a thermal laminate, applied to the surface of the fabric zipper tape, and is prone to delaminating (peeling) over time. The delamination does not interfere with the mechanical operation of the zipper, but it negates the water repellency feature and looks ragged when it starts to peel. Finally, just to be clear, AquaGuard® zippers are "water resistant", not waterproof/watertight.
Bottom line: We prefer regular reverse zippers for their clean appearance, smooth operation and longevity. We appreciate the appeal of AquaGuard® zippers for their "technical" appearance and water repellency, and are happy to offer them as an option. The choice is yours.