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.
Otherwise, everything argues in favor of JavaScript fields...
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.
3. More people know some JavaScript; VERY few people know Memento's proprietary Calculation field script language.
4. Once you learn a little JavaScript, you can use that knowledge elsewhere, within & outside Memento.
5. If you share libraries & scripts with others, more of them are likely to already know the language.
6. Once you learn a few JavaScript functions, the pick lists no longer add much advantage.
7. Once you start learning JavaScript, you tend to continue learning more, extending far beyond the capabilities of Calculation field scripting.
8. You don't need to know much JavaScript at all -- just a tiny fraction of it -- none of all that stuff that's required to code a Web page, which is the main thing it's used for.
9. Once you've exhausted what you want to do with JavaScript fields, you've just barely begun to do what you can do with Memento Trigger & Action scripting, not to mention Data Source scripting.
Also, look at the table of links at the bottom of any of the Scripting pages there, especially the links to Mozilla Developer Network (MDN) JavaScript pages, keeping in mind (again) that you don't need to learn any of that Web page, or Document Object Model (DOM), stuff and not all that much of the other stuff, either.
(I don't recommend you try using an app to learn JavaScript, cuz 95% of what they'll start to lead you through will be the Web page stuff you don't need to learn.)
If you need help on all this, don't hesitate to contact this forum in general or me in particular for customized assistance.