How is Biddy a contrast to Estella and how does Pip perceive both girls?

In Chapter Seventeen, Pip notices that Biddy is turning into a woman, not very pretty, but very bright and wise. They go for a walk and Pip confesses his desire to be a gentleman. He also admits that he wants to be a gentleman so that he will be acceptable, and perhaps loved, by Estella. Biddy wisely suggests that becoming a gentleman to "gain over" a woman who thinks him course and common does not sound very logical.

Pip knows this instinctively, can't help himself and says as much, amidst tears in front of Biddy. He tells Biddy that he wishes he were more easily satisfied, he wishes he could fall in love with her, Biddy. "But you never will, you see," Biddy replies.