What is the advantage of the base 100i model compared to the other variants?

While the 100 series share a lot of their features, the 100i will have a better maneuverability than the cargo variant. The 100i and 125a are very close together but the speed/offensive loadout make the 125a better in combat. That said, as they are all starter ships, those differences wouldn't necessarily be as noticeable as in bigger ships.