Iron Balusters are only available hollow tubular steel. 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. Strength is not an issue. Tubular steel balusters are extremely strong.
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.