As has been pointed out, these are two completely different texts attributed to Zhiyi.
"Essentials" I recall was written by Zhiyi for his brother who was going through a difficult period in his life. Zhiyi was recommending basic Buddhist meditation practice to help him. I think there is another translation of the text in English... I've used it as a meditation manual and recommend it. Another Zhiyi work translated by Dharmamitra, Six Gates to the Sublime Dharma, or something along those lines, is also a very good meditation manual.
Mohozhikuan - which Cleary translates as "Stopping and Seeing" - I do not recommend that work AT ALL. Its abridged and Cleary makes no indication where he has cut parts out. Its very hard to follow. If you are interested in a very good translation of this text - Check out Paul Swanson's, published by Risshokoseikai. It is very well researched and annotated with extensive footnotes. Unfortunately, it is only available as a PDF on DVD. There are sections published on Swanson's pages at the Nanzan University web page. A little googling should get you there. However, Swanson's translation only includes the first 6 chapters on the DVD, and then the beginning of the 7th chapter on his website. A large bulk of the text, the balance of the 7th Chapter, remains to be translated into English.
Mohozhikuan is not a meditation manual the way Essentials and Six Dharma Gates is. Those texts are pretty basic - not breathe-in-breathe-out basic. Maybe better to call it intermediate - how meditation session should proceed in stages, how to deal with obstacles, what the goal is, etc. Mohozhikuan is more of a doctrinal exposition taken to be Zhiyi's crowning achievement. Its said to be an exposition of his own enlightenment. This work is generally read in the context of serious Tientai/Tendai practice, along with Profound Meaning of the Lotus Sutra (a general explanation of Zhiyi's interpretation of the Lotus Sutra) and Words and Phrases of the Lotus Sutra (a commentary on the Lotus Sutra), as well as other doctrinal commentaries on other works like the Vimalakirti Sutra. Basically, this is more appropriate for sectarian Tientai/Tendai/Lotus Dharma traditions and may not have the same resonance with people of other traditions.
Essentials and Six Dharma Gates can be approached by a more general audience, I think.