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.
If you lean toward the sheet, our best cooling sheets list compares four sets by fiber, pocket depth and care from maker specs.
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 Levoit Core 300 vs. Core 200S: Which Air Purifier Should You Buy?.
To widen the decision beyond this head-to-head, How to Choose a Cooling Mattress Pad for Menopausal Sleep Comfort and Best Mattresses for Every Sleep Style provide the broader context.