Just to comment, but as far as I know the "old" ZPP API is not deprecated>
FastZPP inlines the logic for handling params within the function which increases binary size (although it might be negligible), however, union parameter types are only supported through FastZPP.

Oh ok, I believed it was. Thank you clarifying that.

Is there a recommended practice on choosing between FastZPP and ZPP ?

Oh ok, I believed it was. Thank you clarifying that.

Is there a recommended practice on choosing between FastZPP and ZPP ?

Not that I know of, FastZPP is usually used when the function is common, as it can speed up it's execution significantly as ZPP cannot be inlined due to the variable arguments.
Or when union types are involved as those are only supported through FastZPP.

It might make sense to convert everything to fast ZPP but I have no idea if it is that useful :-/

When the fast ZPP has been introduced, I was very reluctant to apply it everywhere (e.g. I applied it only to a few ext/gd functions), but it seems to me that has been applied to so many functions where the performance advantage is not even measurable, so that it is okay to use it everywhere. After all, readability is much better.