When looking at gas water heater reviews some of the things are fairly straightforward - you need to know about the quality of the guarantee (is it long enough and what is covered) and you need to learn a little about the company (are they established and reliable, are their plenty of qualified service technicians).
The ratings tankless hot water heaters should also be contained in any decent consumer review. There will be two main ones, presented in various ways. These two relate to the efficiency of the unit and the hot water heating power.
Gas tankless water heaters in the past got away with quite low efficiencies (because they still offered cheap heating). A unit you purchased ten years ago might only be 60% efficient. That is that only 60 of the gas energy is going to heat the water. The rest is lost to the atmosphere or wherever. It is the hot steam you see escaping from old boiler outlets...
Look for a natural gas tankless water heater to over 90% efficiency or more these days. Technology has been improved so that the unit makes the best of as much of the heat produced as it can. What is good for you ongoing cost control is also good for the environment. These more efficient units will most likely cost more to begin with however.
Natural gas tankless water heaters will also have there heating power expressed in gallons per minute. this is the figure of the maximum amount of hot water the unit can provide at any moment. Buy one that will meet your needs by doing the math on your appliances.