I would say red belt for these reasons:
1. Red belt is the time when you start to seriously think about black belt. As a result, you must begin to develop black belt technique. It is not enough to be able to do the technique, you must be able to do it well. In order for you to test for black belt, you must show you have black belt-level technique.
2. You're instructor does not consider you a beginner, and does not treat you as one. You are treated as an advanced student. He is less forgiving of your mistakes. Previously, if your stances, basics, kicks, and form needed work, it was acceptable because you were a junior student and didn't know any better. Now they must improved.
3. Senior students (black belts) are less merciful. Before, they'd go easy on you because you were a low ranking belt and didn't know to make defense or get back. As a red belt getting ready for black, you tend to be targeted by Dan students who want to see what you are made of and whether you deserve to wear the same belt they do. Free fighting gets more painful around this time. I've seen red belts get bloodied and knocked around.