I've always loved
Sally. My father wanted to name me
Sarah after his late mother, but my mother found a name she preferred, and so do I. But, if they'd thought about
Sally as an alternative, I'd have been very happy. I don't think about
Sally as a nn at all, just a derivative.
Polly is a family name, but only as a nn for
Mary. Once again, I wouldn't expect a modern
Mary to be known as
Polly, but it doesn't seem to be used much as an independent name, unlike
Sally. If I used it, it would be as a full name in its own right, and I do like it very much, but there are other names I like better so realistically
Polly would never get a look in.