A sheet changes the fabric against you. A topper changes the mattress surface and may add substantial cushioning. Thickness does not establish stronger cooling.
Keep the useful part of your bed
If mattress firmness already works, a different sheet or lighter duvet is a smaller change than several inches of foam. If you also want a softer surface, a topper may be useful for that purpose, with cooling performance evaluated separately.
Compare the care burden
Read the sheet’s wash label and the topper’s separate cover and core instructions. A removable cover may be washable while foam is spot-clean only. Measure the extra depth before assuming your fitted sheet and protector will still fit.
What would count as a useful result?
Keep the room and upper bedding similar and assess whether you prefer the change. A cool first touch or gel label is not proof of all-night heat removal. Neither category is a universal winner for hot sleepers. If you need adjustable temperature, a complete powered system is a separate comparison.
See the cooling mattress pad buying guide for manufacturer-linked examples and the differences between fabrics, toppers and powered systems.
See Also
If you are still weighing both sides of this matchup, keep going with How a Cooling Pad May Feel: What to Check at Home, How to Choose a Cooling Mattress Pad for Summer Comfort, and Brown Noise for One Bedroom vs the Whole House.
To widen the decision beyond this head-to-head, Waterproof Bedding for Sweating: Protection Without Hype and Best Mattresses for Every Sleep Style provide the broader context.