Draft the 19yo 5 potential baller.
The rating balls only express the salary. As you know some 5k players are crap (for example a SG with triple 6-7 in JS, JR and OD with lots of skills in 1-2). Sometimes a low B (3,3k wage) can have lots of 5-6 and you can build a great prospect. Also some 2,9k wage players (C+) became good trainees, while some A with 6th man potential end in a sad way. The only fact that the draft gives you is salary, age and potential. And the only trustable info is the age and the potential. If you draft the "mediocre" player you will get some potential for sure.
You can always risk and go to the 5 baller, it can be a 6th man.. or a multirespectable legend (if that happens you will came to Spain to kill me

), but not all the 5 skill rating ballers have great potential, the fact is that lots of 5 skill ballers have a mediocre potential.
P.D. Define mediocre, if you see a C-/C/C+ with 5 balls potential I would leave it for the second round. All the rivals that only scouted once would never pick a guy with 3 balls as the first choice (remember that they need to see twice to have the potential). If what you see is some kind of B with 5 balls I would go straight to pick him.