I have both a PC and a Mac. Which version of GraphPad Prism would I be happier with, and why?

The Windows and Mac versions of Prism are very similar, so you'll be fine no matter which you buy. 

There is one major difference: Prism Windows, but not Mac, allows Object Linking and Embedding (OLE). This is useful in two ways.

When you copy a Prism graph (or layout),and paste into Word or PowerPoint, you can paste an object or link (instead of a picture). Then you can double click on the graph in Word (or PowerPoint), to edit the graph in Prism. After you edit the data, adjust the analysis parameters, or fuss with the formatting,  the revised graph appears back in Word. With Prism Mac, copy and paste just transfers an image. With Prism 4, you could ungroup the graph within the other program. With Prism 5 and newer versions this is impossible (Apple changed the way the clipboard works.)

OLE also works to embed or link Excel worksheets within Prism Windows, so data can flow from Excel to Prism. When you paste a data object (or link) into Prism, you can't edit the data in Prism. Instead, you double click to return to Excel.  With Mac, you just paste the values.

The Windows and Mac version of Prism share the same file format, so you can start work on one platform and switch to the other with no transition hassles.