A water-cooled system circulates water through a compatible bed layer and control unit. Gel-labelled bedding may be foam or another passive construction. Those mechanisms are different, even if both products use cooling in the name.
What active control requires
Check the exact system generation, coverage, controller and required connections. A replacement pad alone is not a functioning cooling system. Verify room requirements, ventilation, approved layering and the water-care routine before comparing price.
What gel does not establish
A gel label does not prove selectable temperature or an all-night cooling result. If the product is foam, evaluate it as a cushioning change too. Do not infer machine washability from a removable textile cover.
Choose for the problem you have
If mattress feel is already right, extra foam may be unwanted. If you want adjustable bed temperature and can manage the equipment, investigate a complete water system. Neither choice is presented as treatment for sleep problems. Use the main cooling buying guide for sourced examples and package checks.
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 Air Purifier for Bedroom vs Dehumidifier for Mold.
To widen the decision beyond this head-to-head, Cooling Pad Fit: Fitted Skirt or Corner Straps? and Best Mattresses for Every Sleep Style provide the broader context.