STAAD III has some basic errors. For instance, it does not take into account the eccentricity of the shear centre of members, so basically you can't analyze asymmetric sections like channels using STAAD. Also, dynamic results are often erroneous.
Check out the frequently asked questions in the "structural engineering other topics" forum. There is a post for structural analysis software evaluation.
What it amounts to is a lot of resources that will allow you/and others at your company to make informed choices.
Sap2000 is by far a better program than STAAD or GTStrudl.
Specially for seismic design. Not to mention the superior Graphic User Interface.
SAP2000 is the commercial version of SAP a research programa developed at UC Berkeley.