Neither fiber is automatically better. Nylon blends are often selected for a smooth hand feel, while polyester blends can suit certain print and chlorine-exposure routes. The exact knit, elastane, finish and construction decide performance.
Start with end use
Resort, fashion, training, surf and kids' products expose fabric to different movement, chemicals, sun and abrasion. Write the use case before comparing swatches.
Test the shell and lining together
Opacity, support, recovery and dry feel change when layers are combined. Review the intended color under stretch and when wet.
Ask how color is created
Piece dye, solution dye and digital or transfer print routes affect appearance, minimums and repeatability. Compare the route, not only the base fiber.
Approve one exact material
Record supplier, article, composition, weight, width, color and finish. A generic phrase such as 'polyester swim fabric' is not a production reference.