How does Prism compare with SigmaPlot?

Of course, the answer depends on how you plan to use the program.  The main advantage of Prism is the data, graphs and analyses are linked. Update or replace the data, and the results all update. This is great for fixing mistakes, but also for running routine analyses.  This means you can make a template easily. Just add new data, and you are done.

Another big difference is, Prism makes it really easy to enter replicate values and plot error bars.

A final difference, is the Help of Prism not only helps you use the program, but it also helps with understanding curve fitting (and other stats), which help scientists learn statistics. SigmaPlot's help is almost entirely for how to use their program.

The best way to compare is to use the programs. Both programs offer free demos.  Download a free trial version of Prism, which gives unlimited use for 30 days.