Piper is superior

She has a cool yellow design and she's even featured on the movie cover/poster with Fender and Rodney

Piper meets Rodney in a natural way and it's easier to believe why she likes him. Nice interactions too.

And Piper is more relatable (ditzy) rather than perfect prissy Cappy. Cappy just likes Rodney for no reason and helps him for no good reason.

Piper is written too immature-ish though, clearly meant to be a teen crush not a full blown love interest or love rivalry.