Look at the efficiency ratings.

In theory, minimizing surface area should increase efficiency. If my high school geometry is getting recalled okay, a cylinder of a given volume will have a smaller surface area if it is shorter and wider. However, that doesn't account for, for example, heat rising (meaning a bigger top surface is worse), or individual design characteristics.

It is also a fact that short water heaters will deliver more hot water on first draw due to wider vessel allowing for more draw from top before the cold water incomming stratifies the entire tank.

In addition larger diameter tank bottom increases surface area for burner contact, shorter flue up center decreases chimmney effect cooling tank when burner is off.