The difference between the two units is the annode rod inside the water heater. The annode rod absorbs the electrolysis that takes place on metalic parts inside the water heater. On a 12 year model the annode rod is larger so it lasts longer. An annode rod is called a sacrificial metal. As water passes over it, it exchanges electrons with the charged particles in the water. Since its a softer metal than the iron tank of the water heater, it breaks down first. With the smaller rod, after about six years, the rod is so pitted, it can no longer function. So any minute exposed metal surfaces start to break down, developing a hole in the tank.
Even though most surfaces inside the water heater are coated with a fiberglass coating, they cannot cover all the welds and openings welded on for the inlets and outlets of the water heater. These are the surfaces that break down.
If you change out the annode rod after six years with another one, it will last longer. So it really depends on your ability to maintain the water heater. If you are handy, then buy the cheeper model. If not, the 12 year is a better value.