What Rob is saying is that there is not a straightforward answer. It depends on your problem and what results you are trying to obtain. Fluent, Polyflow and Mechanical are all tools under the ANSYS simulation group. Just as a hammer, a screwdriver and a saw are different physical tools, these softwares have been developed to do certain jobs. Are you sure Fluent is the right tool and not one of the others? You would be working on the limits of what any of these programs can do.