When the variable is a static value (like apple, 10, etc)
The biggest difference is that the one in the script will change to the value specified in the script (unless within an if statement) whereas once that are on the front end won't change based on a refresh. You can always create the variable in the script and remove them after creating them from avoiding to reset there value everytime you run your script.
When variable is an expression: Sum(Sales)
Its just a matter of preference, I sometimes do it in scrip, or even in an Excel file (Storing variables outside of QlikView - The Qlik Fix! The Qlik Fix!). But mostly on the front end for no specific reason.
