You can see the difference in growing up in this chapter
Main series:
Iruma got used by his parent till his teenage and just at 14 he is adopted by loving and rich grandpa. Makes him having instictual wariness, but still naivity from not experiencing the life of demon world from young age.
Mafia IF:
Iruma is adopted since 4~5 years old and grow up with Mafia education, makes him know the inner working and darkness of the side yet still able to keep his innate kindness and hope.
Both are a nice way to see Iruma, and from Mafia Iruma, we can see how Iruma will be when he become the demon lord with all the education it bring: someone reliable who can plan his way through his want withut sacrificing his ideal and hope.