Currently we use a Fiberglass tank by Flexcon. These tanks are becoming increasingly popular because there is no steel to cause them to rust out. If the tank is setting next to a water softner, the salt spray will not rust out the firberglass. These tanks have a strong bladder inside and come with a five year guarantee.
Tanks come in a variety of sizes. Generally they are matched up with the pump installed and water usage to cut down the on/off time of the pump for lasting equipment. Other factors when selecting the right size tank for you is the amount of water you will be using. A larger house with several bathrooms or if a sprinking system is going to be used, a larger tank is recommended.