While fiberglass tanks are less expensive, concrete is not subject to damage by UV radiation. Over time, you can expect the fiberglass tanks to become more brittle. However, if you reside in a very windy area, fiberglass tanks are less desirable especially when first being filled or are empty due to cleaning. An area school had an above-ground metal tank delivered to the property. This could easily happen to empty fiberglass tank as well but this situation can be avoided by securing these tanks when empty.
Stainless steel are a good choice but fairly expensive by comparison.
Some are portable and can even be loaded on the back of pick-up trucks. The amount of rainwater harvested depends on the total surface collection area, the volume of storage, and the amount of rainfall. Newer storage tank models also use the roof of the cistern as catchment area and come equipped with their own guttering feeding directly into the tank. The maintenance of the free chlorine residual will prevent establishment of any microorganisms. The maintenance level should be kept about 3-5ppm of free chlorine.