There is no fixed winner between the two fibre names. Whether a brief "bags out" depends on how much the knitted fabric stretches under wear and how fully it recovers. Spandex content, yarn, knit construction and finishing all affect that. A cotton-spandex and a modal-spandex jersey can each be made to recover well or poorly. The useful step is to compare the two fabrics you have actually been offered, with a test.

Turn "bagging" into a measurable property

For knitted fabrics with low power, the kind used in underwear bodies, ASTM D2594 measures stretch properties including fabric growth, which is the stretch that does not recover after the fabric has been held extended. That is very close to what a customer describes as sagging. Ask the factory or a lab to run both fabric options under the same method and compare growth and recovery, rather than relying on handfeel at the swatch stage.

Also ask for the exact fibre composition and construction for each option. "Modal" and "cotton" each cover a range of yarns and weights, and the spandex percentage in each offer may differ. A comparison only holds if you know what was tested.

Consider the waistband and leg openings too

Some complaints that sound like body-fabric bagging come from elastic at the waist or legs relaxing. Those trims can be tested separately, for example by ASTM D4964 for elastic tension and elongation. Read the reviews closely. If customers say the waist rolls or the legs ride up, the elastic may matter more than the body fabric.

Choose with washed results

Recovery can change after laundering, so test both options after an agreed number of cycles under a named procedure such as ISO 6330. Then choose using the washed results alongside comfort, price and your brand positioning. Modal is often chosen for softness, cotton for a familiar feel. Neither preference replaces the recovery data.

Once you have chosen, write the growth or recovery result into the fabric specification. Then bulk fabric is checked against the same property your customers complained about.

A practical sequence is: shortlist both fabrics, test growth and recovery new and washed, sew a small fit run in the better performer, and wear-test it before committing the full colour range.