Birds prefer a rough surface that won't cause them to slip. I think concrete is an ideal surface, but you can also make a birdbath less slippery by covering it with those stickers they sell for bathtubs. In climates where water freezes in winter, birds need thawed water on cold days. The easiest way to provide reliable thawed water is to use a submersible, thermostatically controlled heater. Another consideration in choosing a birdbath is size.
Small birdbaths tend to be used by one bird at a time.
However, unless you're determined to see a lot of birds splash around at once, the small baths do work and are likely to be less expensive. Another issue is style. For stability and a natural look, I'd go with either a concrete bath or a dipping pool made with a plastic liner and pebbles. If you're concerned about keeping that type of bath clean, however, look into the styles with the convenient removable plastic bowls. Plastic can be slippery and the bowls may have steep sides, so you may have to add rocks to a bath of this type.