Qui-Gon didn't choose to become one with the force, he was killed by Maul, while Obi-Wan, Yoda and apparantly Anakin all disappeared and became one with the Force before they actually perished, that's why they could come back as apparitions, while Qui-Gon could only come back as a disembodied voice.
Before the prequels, I subscribed to the idea that it was only Luke who could see them through the Force.