The Zohar describes the relationship between the Choshen and the Ephod using the verseאחור וקדם צרתני, which can mean “You have fashioned me front and back.” The Ephod worn at the back and the Choshen worn at the front, represent 2 different pathways of serving Hashem. The difference between them is analogous to difference between the back of a person and their front.
When looking at someone face-to-face, we see animation and expression, which convey their emotions. In contrast, when looking at someone’s back, we cannot discern any emotion.
A face-to-face relationship with Hashem is one that is permeated with feeling and desire. The word for face (Panim), is related to the word Penim, meaning inner. This inner service is represented by the Choshen, which was worn at the front and over the heart, the seat of emotion.
A “back-sided” relationship with Hashem represents our service when we lack feeling. In Kabbalah, this Avodah is called Iskafia, compelling. When we are not inspired and we lack feeling, we must use our mind to dominate the heart, to compel ourselves to serve Hashem even when our heart is not in it. This service is represented by the Ephod.
These 2 pathways are the different services of Tzaddikim and Beinonim. The Tzaddik is constantly alight with fiery love and passion, always inspired to serve Hashem with an inner service. The Beinoni struggles with physical desires, dullness of heart and lack of inspiration which they must overcome to serve Hashem.
Even though one was worn at the back and the other over the heart, both the Choshen and Ephod are holy and both are integral parts of the Kohen Gadol’s clothing. If either were missing, his Temple service would be disqualified.
Hashem formed a unique few “front”, to serve Him without struggle and with inner feeling. Most of us He formed “back”, with inner battles that we need to confront and overcome. Neither Avodah is superior to the other, they are both holy garments and Hashem has Nachas from both, as long as we apply ourselves to the service that He has destined for us.