Fe-Si vs Si-Fe

Before we start, I'd want to summarize both Fe and Si:

Fe: "Extraverted feeling is based upon accepted or traditional social values and opinions. It involves a conforming, adjusting response to objective circumstances that strives for harmonious relations with the world."

Si: "Introverted sensation is subjectively filtered. Perception is not based directly on the object, but is merely suggested by it."

Fe-Si: It mainly judges the world using collective and societal values, it strives to build a community that is suited for the majority of people, then it will see if it aligns with their own subjective impression of the object. They will lay down the relationships, the values and the emotions then they will check if it matches with their own personal perception of the object matches with their judgements. 

Si-Fe: It mainly perceives the world using subjective impressions and experiences, they will first refer to their own personal experience and then relate it to the emotional consensus, it focuses first on their own personal impression of the object but then back it up with societal values.

Compare and Contrast: Both functions mainly refer both to their subjective impression and the collective value, however the difference is that Fe-Si mainly prioritizes relationships and societal values more and just uses their own subjective impression as a support and secondary reason for their action, while Si-Fe mainly prioritizes subjective impressions and experiences more but then needs the emotional consensus to back up their personal experience/impression.