Birdbaths come in all shapes, sizes, materials and price ranges. From the totally elaborate molded concrete "art forms", to the fancy cast-iron filigreed, to the purely-functional plastic pedestal & saucer.
So, buying a birdbath becomes an integrated decision...i.e., based on the following:
What is your "purpose" for buying a birdbath?
If it is simply to provide a place for birds to bathe, (with minimal priority for aesthetics or landscaping appeal), then BUY PLASTIC!
And, if you fill the pedestal section with sand or stones, it is less likely to be tipped over by the midnight drinkers that may find it (deer, raccoons, bears?).
If it is to provide a place for birds to bathe, PLUS provide enhancement to landscaping appeal, consider some of the cast-iron (and/or plastic) decorative type. Some of these are quite elaborate and ornate in design, and surely add eye appeal. However, some are difficult to keep upright, especially in rural areas where other wild animals may find it. One trick to prevent tipping is to use "ground staples" (available at most home centers) if the cast base piece has places where they can be utilized (i.e., many bases are filigreed, allowing staples to straddle some of the artwork, thus anchoring the base section to the ground).
If it is mainly just to provide enhancements to landscaping appeal, but you still want to provide a place for birds to bathe, consider a birdbath/fountain unit. There are many styles to choose from (faux-stone, concrete, cast-iron, plastic), and they always add restive beauty to any yard. Keep in mind that placement is critical, as fountains require electrical power to operate (unless powered by solar energy).
To prevent stagnating water, bird baths must be kept clean, and water must be replaced frequently, even daily if possible.
Water in larger fountains may need to be treated if it is infeasible to replace it. Look for treatment products that is certified to be safe for wildlife.
No matter what type of birdbath you decide on, please remember that the water will be consumed by innocent little birds, as well as other animals.
Birds can drown in birdbaths if the water is too deep or there are no shallow 'perching' areas.
Attracting birds to your birdbath can also attract larger birds of prey or other animals that feed on birds.
Poorly maintained birdbaths can become mosquito breeding-pools and can be harmful to birds or other wildlife, and even to humans. If you're not going to maintain it, please don't get it!
Locate your birdbath near bushes or trees, so birds have a safe place to escape to, if necessary. Keep in mind that when their feathers are wet, birds do not fly as well as when they're dry, so if they need to get to the protection of bushes or trees in a hurry, the closer the better!
Direct sunlight on your birdbath will accelerate evaporation and stagnation; keep your birdbath in a shady spot, if possible.
Good luck, and enjoy!
Image courtesy of Paul Brentnall at freedigitalphotos.net