What I have seen
As a practical matter the so=called "value line" paints do not have as much "stuff" to them and do not cover as well or flow out as well and in the end do not hold up as well..The premium paints cover well and work well for the painter..and do hold up better IMO saving a lot of time and trouble..By not putting "stuff' in the paint or skimping in the paint mix the paint maker can save a few bucks which is then passed to the customer..so there is a difference in paint quality..
In the cost of a paint job the paint itself is a small part of the expense the major piece is the labor involved in getting a good job..Saving a couple of bucks on paint is more than wasted in additional labor and time making cheap paint "work well"
One of the things I can get "positively anal" about is using the proper hardeners reducers and activators that go with the particular paint line that is being used..Even if you are using paint from one of the majors it does not necessaruly follow that the activtors and hardeners for say the Nason line would be the same for the premium line even tho they came from the same company..
One of the reasons a collision shop will settle on using a particular line of paint is so the painters do not play chemist either on purpose or by accident and create a mess that results in a bunch of callbacks..Say by using a paint from one company with some sort of additive or reducer from another..
My thoughts on getting a quality job..
I have tried most all of it and now do what is known to work..