Gas water heaters come with an energy rating label: the more stars on the label, the more efficient the heater. There are standard and high-efficiency gas storage heaters. High-efficiency models are more expensive to purchase, but cheaper and more environmentally friendly to run.
Gas storage heaters only need relatively small tanks, as gas is available 24 hours a day and heat losses can be replaced quickly. A four-person household needs a tank of about 135–170L. Gas heaters can be installed externally, or internally with a flue.
Remember though your system and settings need to compliment your lifestyle. A household with teenagers and children will place greater demand on a hot water system than a household with a couple only.