The teaching of the Buddha-nature immanent in all things was not simply a matter of two approaches, sudden vs. gradual.
Different teachers dealt in different ways with the practical implications of this teaching, and in particular with the question of how to regard the defilements of the mind in light of the Buddha-nature's supposedly being intrinsically pure.
Shenxiu approached the problem from two angles. When describing the practice from the outside, he stated that the pure mind and the defiled mind, though conjoined, were essentially separate, each with its own intrinsic reality. Neither generated the other. Thus the goal of the practice was to rid the mirror-like pure mind of any impurities. When describing the techniques used to rid the mind of its impurities, however, he recommended that the meditator regard the impurities as essentially unreal. In reality, some of the practices he taught implied a sudden approach to awakening; others a more gradual approach.
For different folks . . .
In this view, they are not so much stages of spiritual life as streams flowing from the self nature as the sole fountain of wisdom. Everything depends upon awakening. The self-awakened will spontaneously avoid all evil and pursue all good. He enjoys an ineffable freedom and peace, and carries within him a living fountain of wisdom.
The way of Huineng was, then, professedly opened for men of the highest spiritual gifts. But taking mankind as it is, one can not but admit that even "men of Mahayana" are rare enough, to say nothing of "men of the highest Vehicle." One wonders how many monks and lay devotees who have claimed themselves to be members of the southern school of instantaneous enlightenment have in truth been men of the supreme vehicle as envisaged by Huineng.
Rather than two separate paths to realization, the two "approaches" arise from the different capabilities and degrees of commitment of practitioners. The gradual path of meditation, study, discipline and good works suits certain people; others are better served by practicing "no thought" and seeking a sudden breakthrough to enlightened consciousness. Looked at from this vantage point, the two paths represent different expedient means.