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How about lifetime costs? A resistive water heater is going to last longer and you can get non-metallic water heaters for extremely long life.

If you don't have net metering (or just a terrible power purchase rate), why not just sink that extra solar energy into a water heater?

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That is a good question. The usable life of a combined heat pump (space heating/cooling + DHW) is somewhere between 10-15 years. For the ones, dedicated to DHW, I cannot really tell, but I would expect it to be longer, because they are usually less heavily loaded. Then the answer of your question will depend on the price of installation (e.g. heat pump vs 3 x extra PVs), local climate (affects efficiency) and price of electricity. Heat pumps don't work well in very cold climates, but for most US/EU cities they will work fine. For very hot climates it may be better to use solar water heaters, although for commercial installations there is the option to use the exhaust heat from cooling to heat DHW (it is essentially free energy).

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