I am well aware of the Diana/Florence "battle" over "People." On their recorded version, Florence sings lead on the first two verses, Diana sings lead on the last and they close with truly gorgeous three-way harmony. For my money, Florence is much better on harmonizing than singing lead; it's only when Diana takes the lead that the lyrics seem to mean something and actually land. But there are live versions available on YouTube in which Flo sounds far more assured.
Diana is sensational throughout "Diana Ross and The Supremes Sing and Perform Funny Girl."