It depends on what you want to do. STAAD is cheaper, and geared more towards buildings. SAP is a much better product for bridges, seismic, nonlinear.
I use both products. I disagree with fleisher, in that STAAD has given me more suspicious results, and crashed more often than SAP.
The big user difference between the two is the way input files are stored. STAAD is a text file, which offers much comfort to those used to working in text files and commenting text files. SAP by default stores the input file in a database. To edit you must either use Excel, Access, or the SAP program.