Web application is a client-server application in which the system responds to requests from clients.

The client is every browser and other application that is installed on the user side, what is sometimes called front end.

The server side responds to all requests that come as basic paths or URLs with arguments or even JSON.

Which is the preferred way to get the current request and why? Is one of these to be deprecated? Any other advantages/disadvantages?

When building a new module, you always get to the point where you want to use the routes you have been created, or other URLs, so if you find yourself always looking for how to print out a URL, this article is for you.

The difficulty is that finding or printing out a URL is very contextual and there is more than one way to get or use this information in Drupal. 

You might have a node object that you need to convert into a fully qualified path, or you want to print out the path of a route, each of which has different approaches.