They are both way to expensive to consider if you are a typical hot water user, but the propane will be a even more expensive than nat. gas.

If your asking: "can I hook-up a natural gas water heater to a propane supply?" the answer is NO. However, there probibly is a conversion kit available from the manufacturer.

no I'm deciding which is better to get with the corresponding supply source.

If you have NG use it.

It typically has the lowest cost.

The only savings you might have with a tankless are the standby losses as the tank cools off and needs heat to maintain the water temperature.

This usually pales in comparison to the initial cost of heating the water up.

NG is used to generate electricity, propane never is (high cost).

thanks