Fi vs Ti, in a nutshell
An ENFP will use their personal value system to make decisions whereas an ENTP goes back on their personal logic system, as such. In practice, you'll see an ENTP arguing hypotheses, interpretation of facts, statistics on a situation whereas an ENFP will argue morality, personal freedom of choice, diversity, unique perspectives, etc.
Additionally, theres the tertiary Te vs Fe thing
Enfps, when they do use logic, will use empirical data, facts, logistics and organisation, whereas ENTPs, when they do use social graces as such, will stick to group dynamics, social protocol and 'gaming the social system' of give and take in society.