I'm not sure that these two are meant to be character foils but there are differences. Much like his beloved Jane, Charles Bingley is an amiable and good-tempered person, mostly unconcerned with class differences despite his extraordinary wealth. His virtue proves to be his vice at times, since his modesty leads him to be easily swayed by the opinions of others. A mostly static character, Bingley remains pleasant and in love with Jane throughout the novel.
Mr Darcy is more thoughtful and opinionated than Mr. Bingley. He is certainly not as laid back. Mr. Darcy is proud, haughty and extremely conscious of class differences (at least at the beginning of the novel). He does, however, have a strong sense of honor and virtue and a degree of fairness that helps him to control his pride after Elizabeth rebukes him for his narrow-minded perspective.