Two different people of any one race tend to look a lot alike unless you are a member of that race, bringing you to insist that nobody could mistake one for the other. Similarly, judging the similarity of two cars is very subjective.
Both come in a silver finish. Both have four wheels, though the Prius wheels look proportionately smaller and it lacks the Insight's telltale fender skirts. The profile of both cars is similar, though the Prius rise to roof level is a bit more abrupt in front.
Viewed from above, or from the front or back, the Prius is a lot blockier. It has corners where the Insight has curves.
Meanwhile, the Prius is a fine car. If I wanted to drive a car with an automatic transmission, I'd consider it, but I don't, so the issue dies there.
Compared to the older Prius model, the new one is very sleek and pretty. The engineering is clearly better in the newer model. This is cause for celebration. The world needs more cars like this.