I'm certainly no expert on either Phillis Wheatley or Anne Bradstreet, but I have read several selections by each of them. As I see no one else has answered your question, I'll share my observations and hope you find them helpful. Bradstreet's work is a little older than Wheatley's, and it was written as private work which was never intended to be published. Wheatley, on the other hand, is a much more sophisticated poet. While Bradstreet was a devout believer and early settler in America, Wheatley was a black woman who had clearly been given some formal educational training.