The choice depends on the customer’s main use. In general, polyester absorbs little water and dries faster, so it suits sweat-heavy use. Cotton absorbs more, tends to feel softer against skin and is familiar to most buyers. Elastane in either gives stretch and shape. But the actual result depends on yarn, knit and finish, so decide with tests and wear trials.

Moisture and drying

Use wear trials, not claims: have testers do the same workout in both fabrics and compare how wet, cold or clingy each feels, and how long each takes to dry on a line. If the mill gives wicking claims, ask for the test method and the report.

Feel and fit

Compare the fabrics worn next to skin for an hour or more. Check stretch and recovery with a method for elastic fabrics such as ASTM D4964, before and after washing, to see which keeps its shape. Check how the leg openings and waistband behave on each.

Pilling and wear

Friction between the thighs causes pilling. Test pilling with a Martindale method such as ISO 12945-2 on washed fabric, and compare both fabrics. Synthetic blends can pill and hold odour in some cases, and cotton blends can lose shape faster; test both.

Washing and drying

Wash both fabrics with a procedure such as ISO 6330 and measure the dimensional change, then decide on care instructions based on the results. Elastane needs gentler heat, so check what drying the garment tolerates.

Fibre label

Under the FTC Textile Rules, 16 CFR Part 303, give the fibre names and percentages by weight on the label, with the main fibres first. The label must match the fabric report, including the elastane percentage.

Decision

If your customer is a gym-goer, trial polyester first; for all-day wear, trial cotton. A range with both, clearly labelled by use, may serve better than one style trying to do both. Keep the product page honest about trade-offs: say what each fabric is good at and avoid performance claims you have not seen in testing, because the same customer who values dryness will also notice if the waistband loosens or the legs pill after a month of weekly washing.