Before purchasing a water heater you should take it to your consideration: Look for one that's energy efficient. You should look for estimated energy consumption and estimated yearly operating costs. You should even look for an expensive one because after the calculation it could be more lasting and would save you the money in the long run.
Picking a water tank, you should know "approximately" how much water you use per hour. You need to know because if you get a larger water tank it will use more energy to heat up the water, and if you choose smaller one, it will save you money on energy.
Tankless water heating systems. Thinking about getting one, you need to know that it is energy efficient. Tankless water heater don't have a storage for hot water. When you turn on the water it heats up right away. But the weak point in this, is that the heater can not produce large ammount of hot water at once. Before buying make sure that you wil have enough for dishwasher and taking shower. You need to know this because if you don't you will be taking cold shower. It's good for you health :)