Okay... you have to balance a number of things but generally here's how it goes:

Most expensive fuel: Electricity running Resistance water heater. Also LP gas

Least cost effective heater: Electric whole house tankless

Gas is cheapest source of energy
Gas 92% tankless can be a very good solution

here's a compromise that could work for you in Texas since you don't have any other appliances that use gas:
Consider an electric heat pump water heater (Hybrid). It pulls heat from the surrounding air to heat the water and can have similar costs to gas. It's a bad solution in cold climates, but could work well in Texas.

Also consider Grid Tie Solar PV. You would never use solar pv to run electric resistance water heater, but does make sense to run heat pump heater (as well as potentially breaking even on your electric usage). Solar PV really needs to be engineered from beginning - so you orient the house right etc.