Both methods are problematic.
If we choose WELCOME_TO_USE_THIS_APP but then decide to change the message, to for example "Thanks for choosing our app" the key no longer fits
If we choose WELCOME_MESSAGE and use it in more than one place it will soon become WELCOME_MESSAGE and WELCOME_MESSAGE_2 and then WELCOME_MESSAGE_3 as we have slightly different versions in different contexts. perhaps "Welcome to choose this app" and "welcome to choose this application." etc
I think the best approach is to be very specific and have a one to one mapping to a particular View or Page
This way our keys should only change when the view that uses them changes, and the text can be chosen specifically with the constraints of the view in mind, such as character limit, font size etc.