A perhaps better answer is don't bother. Just install R (which is free) which works perfectly well on Macs, PCs and Unix systems. Seriously good psychometrics are available as packages in R. These include the lavaan, sem, and OpenMx packages if you want to do confirmatory factor analysis or structural equation modeling. ltm and others for doing IRT based analyses, and my psych package if you want to do exploratory factor analysis, basic reliability (including alpha, beta, and omega coefficients). Just search for R CRAN.
For help on using R in psychology -- particularly in personality, visit http://personality-project.org/r