Water heaters for single family rentals

[James Ross]

Hello! My wife and I are renovating our house in order to eventually rent it out. The water heater's bed pan is rusting through and the water heater needs replacing. We live in Colorado Springs which has fairly hard water. 

Currently we have a 50 gallon gas heater. We're considering whether to get another 50 gallon or going to a 40 gallon or possibly a tankless water heater. Does anyone have any suggestions on water heaters for future rentals? Especially with hard water.

Thank you!