I guess I'm in a position to answer this one.
From my perspective, the experience of realization of Anatta is simpler and less profound. At some point in meditation or in the middle of some other activity accompanied by contemplation of the Twelve Nidanas, or the conditioned nature of all dharmas (the truth of impermanence) etc., one may realize that:
- the thoughts follow a cyclic pattern of free association and do not come from an "I",
- identification with an individual living organism is pretty arbitrary and relies on a rather conventional delineation of boundaries,
- the universe is fully deterministic,
- an immortal soul cannot exist unless it exists in absolute isolation from all interactions with anything else, which is nonsense,
As these conceptual understandings all come together and connect with first-hand meditation experience, there comes a moment when the notion of "I" no longer applies as anything but a designation, a convenient label. That's Anatta.
Then there is an intermediate stage, at least in my experience, when the notion of Anatta of sentient beings is generalized along the lines of the following realizations:
- all entities are mind-made abstractions.
- as times goes by and things transform, in any object there is no identity that stays the same through all stages of transformation. Any such identity is a mind-made overlay.
the above realizations arrive at a point when all inanimate objects and not just sentient beings are perceived as lacking any substance, or empty of any identity, and are mere conceptual designations.
And then... and then... there is the actual experience of Shunyata, which goes waaaaaaaaaaaaaaay beyond anything like the above. The experience of Shunyata comes only from very consistent and thorough research into all topics of Dharma, diligent application of Dharma in real life, and very sincere and truthful meditation.
The experience of Shunyata is the experience of loss of existential ground. It changes everything without really changing anything. It shatters one into a million pieces, then assembles back into a sentient being and abandons. From this moment on, the worst has happened and nothing can hurt you anymore. The world will never be the same.
Naturally, as I belong to a Mahayana school, I see Anatta as incomplete realization and Shunyata as fully matured. The two are really just phases of the same realization as it deepens through study and practice. From this standpoint, if you were to stop at Anatta and never go to Shunyata (never jump off the cliff) you would still have a ground under your feet, which would A) make you still be a jerk to people who disagree, since you'd still have an attachment, and B) due to the same attachment, make you vulnerable to conditions, hence no Nirvana for you.