Iron Balusters are available in either solid wrought iron or hollow tubular steel. Some iron balusters are only available in solid and some only hollow. The iron stair balusters market has been trending toward hollow for the last several years. The primary reasons for this migration are: they are less expensive, cheaper to ship, and easier to cut.
Regardless of which construction type you choose, please make sure your iron baluster installation will pass code. Typically, 3 balusters per stair tread are required to meet the 4″ sphere spacing test which requires a 4″ sphere cannot pass through any part of the balustrade. Always consult your local building department.