Choosing to help Tifa first will increase your odds of getting her semi-romantic scene in chapter 14, whereas coming to the aid of Aerith will increase your odds of having a romantic chat with her instead.

Is Aerith in love with Zack?

Aerith definitely prefers Cloud over Zack. When Cloud asks Aerith about her boyfriend in the original FF7, she replies I liked him (Zack). It wasnt serious. In the Remake she says that Zack was the first guy she ever loved, implying in both games that shes done with that relationship.