Look at the following diagram. It illustrates the relationship between Test-Driven Development (TDD) and Acceptance Test-Driven Development (ATDD):
Notice that the inner loop, TDD, is enveloped by the outer loop, ATDD. Requirements (illustrated by the Acceptance Criteria box) drive the creation of automated acceptance tests, which in turn drive the creation of Unit Tests for implementation.
In general, RSpec would be used in those situations where you want to express tests in a Business Analyst-friendly way, namely the creation of automated acceptance tests (the tan circle); whereas Unit Test frameworks like nUnit are more specialized for the sort of unit tests written from the developer's TDD perspective at the implementation level (the yellow circle).