How to decide between 'Hook_preprocessor' or using twig template? We all know that for a customs web designing issue there are many solutions and various methods and ways,
but this question has always existed, i.e about what the advantages/disadvantages (pros/cons) of each of them are.

For example, by using hook_preprocess in the theme file we can add our desired styles to custom blocks or regions,
and reduce our needs to keeping multiple twig templates compared to times we use twig template to add our styles.

I want to be able to use those criteria (pros/cons) to decide which one is recommended in specific situations by considering indicators such as:


Stability 
Efficiency and Loading speed 
Maintenance
It's not really an either/or thing. They are used together - preprocess functions are run before a template is used. Templates should only have very simple logic, if/else, and loops for the most part. Any logic more complex than this should go into a preprocess function. If you want to do things like adding classes to the attributes of a variable, this should generally be done in the preprocess function, though if you have a class that will only go in a single template, then it can be done in the template itself.