Contractor payroll software focuses on paying independent service providers against authorized amounts. Small business payroll software generally focuses on employee pay processing for smaller organizations, often with related administration and sometimes with contractor-payment features included. The categories describe different aspects of a product: one emphasizes the payment relationship, while the other emphasizes the customer and employee-payroll scope.
A small business can need both capabilities. A company paying a consultant’s invoice and running employee payroll should evaluate each process separately, even if both appear in one application. The important distinction is what the service calculates, records, and handles for each kind of payment, rather than whether the business is small.