Galvanized wire is the standard while plastic-coated galvanized will last longer.
Some baskets have a curved bottom while others have a flat one.
The flat ones have the added advantage of being usable on the deck to hold spring bulbs or flowers set in sheet moss. Because there is very little soil compared to roots in hanging baskets, it is important to ensure that plants are nourished all season. These are not strictly necessary but from my experience these little granules really help reduce watering. Hanging baskets dry out very quickly because they are exposed on all sides to drying winds. In the heat of the summer it is not uncommon to water moss baskets twice a day.