I think Calculation fields execute a bit faster, but I'm not sure. Also, you can pick fields & functions from lists, which makes things easier when you don't already know how to do something.
1. It might very well be just as fast.
2. Using Calculation field functions can be very complex & difficult, once you start doing things beyond the most basic.
5. If you share libraries & scripts with others, more of them are likely to already know the language.
If you need help on all this, don't hesitate to contact this forum in general or me in particular for customized assistance.