Buy the size that you think you need. If you use a lot of water, buy a bigger one, but if your hot water consumption is not that high, buy a smaller one. There is no sense in heating more water than you need. Then choose one with the longest warranty. No one wants to replace a water heater too often. I had to replace mine a couple of years ago. It had a very very slow leak and at first I thought that my very dry basement was becoming damp. Then it really leaked. We have a Maytag. I joked at the time that it was the only Maytag appliance we had. Now I have the washer and dryer. It was the one that the oil burner company that I use recommended. The cost of the water heater isn't so bad, its the installation.