There may be a third option, but it would depend on your future plans. It also has the disadvantage of there not being a fuel backup if your electric grid went down. Many people are trying to go as electric as possible because they can subsidize their electricity use with PV power. This would have the same effect as zeroing out the future expenses for water heating. That's if New Mexico is a grid tied state?
I personally plan on going that route. I've already bought an electric Marathon water heater which is a fiberglass design and is supposed to last a lifetime because it doesn't have the usual rusting out problem. They are about a thousand dollars. I think it is wise to think not only about the grid going down and causing cold showers. Cold showers can also be caused by mechanical failure. Solar hot water heaters have a very poor track record in this regard. Also regular metal water heaters last only about 10 years and then rust out. So a Marathon fed by PV is a very viable option compared to what you are considering. Of course, PV can be used for a thousand other things also which makes it more flexible even than cheap natural gas.
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