100% role queue. Random don't have the coordination to assign positions and build a team in the short amount of time you have. Also with role queue you can really learn the ins and outs of a specific role which will help you climb faster. Just learn 1-2 heroes in all other roles when you have have to queue for all.
That said, if you do queue for all, you almost always get 3, 4 or 5.
Role queue, but for another reason than just "people get the role they queued for".
With role queue, you get individual MMR adjustment based on which role you're playing. Or in other words, your performance with each role gets incorporated into the search algorithm. You don't have that with classic ranked.
As a result, you get fairer games. Let's say for example that your displayed MMR is 2000, and you are worse as a pos 2 than a pos 1. When you queue and get selected as pos 1, your MMR may be counted as 2100, i.e. you get stronger enemies or worse teammates. While when you get selected as pos 2, your MMR may be counted as 1900, which means you get worse enemies/better teammates, to account for you usually playing worse as pos 2.
I think this is a much more important and noticeable factor than just the automatic role selection, as I would suspect that at this point, whoever plays classic is doing that because they don't mind figuring out the roles in game. However I can not confirm that as I'm only playing role queue because of the above reason.