I am going to develop a multiuser payroll application. It will developed to an enterprise level. Initially it will be used by an organization but in future it may be marketed as a product.

But I can't decide what back end database I should adopt - Oracle or SQL Server?

Will anyone help me how to decide on this most crucial part? What factors should I consider? Assume that I have equally competent programmers in both Oracle and SQL Server.

Thanks in advance.