The answer to 'water heater vs' is that water heaters and furnaces/boilers are both essential home heating systems, but they have different features, benefits, costs, and use cases. Water heaters are designed specifically to heat and store hot water for household use, while furnaces and boilers provide whole-home heating through a forced air or hydronic (water-based) distribution system.
Water heaters are generally less expensive to install than furnaces/boilers, but they have higher ongoing energy costs to continuously maintain hot water. Furnaces and boilers have higher upfront installation costs but can be more energy-efficient for whole-home heating, especially in colder climates. The best choice depends on your home's size, energy needs, and budget.