Even though the types only differ by one letter, they actually have big differences. And INFJ is the rarest type while ISFJ is the second-most common.
The biggest difference is in dominant function of Introverted Intuition vs Introverted Sensing.
ISFJs are leagues more detail-oriented and consistent than INFJs, who are notorious for missing details because Sensing is the Inferior function for them.
ISFJs respect the past and tradition. INFJs want to change things for the better, and tend to be highly future-oriented and see the past in a negative light.
ISFJs have a good memory but a weak ability to see underlying meanings. INFJs excel at finding the hidden meaning, but often have quite a poor memory.
Under extreme stress, an ISFJ will catastrophize about all the possibilities that could go wrong. Under extreme stress, an INFJ will lose their cautious nature and become suddenly impulsive.
ISFJs have more patience for small talk.
ISFJs rely on concrete information, while INFJs rely on abstract impressions.
ISFJs help others by caring for them directly, while INFJs help others by advocating for them indirectly.
ISFJs are slightly more emotional than INFJs, even though both types have a balance of emotion and logic due the Feeling and Thinking functions being in the middle.