Phase-change material describes one feature; topper and pad describe broader constructions. A thick topper with PCM and a thin protective pad can perform different jobs even if both are advertised as cooling.
Decide whether you want extra cushioning
If the mattress already feels comfortable, a thick topper may introduce an unwanted feel and height change. If more cushioning is wanted, inspect the actual filling and thickness rather than attributing the comfort change to PCM alone.
Measure total mattress depth before assuming your sheet and protector will fit. Read the cover and core care instructions separately. A thicker item is not automatically washable, and a thin pad is not automatically waterproof.
Check the cooling evidence separately
PCM stores and releases heat during a phase change, but its label alone does not establish how long a finished product will buffer heat under your bedding. See gel and phase-change material limits for the mechanism and evidence boundary.
There is no tested average-sleeper winner here. Choose the construction that solves the actual comfort or protection problem, then use exact product documentation and return terms to assess any cooling claim.
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 Sleep Humidifier vs. Sleep Sound Machine.
To widen the decision beyond this head-to-head, Queen Cooling Mattress Pads: Passive Pad or Active System? and Best Mattresses for Every Sleep Style provide the broader context.