Ruby mocking frameworks have evolved a lot since this question has been asked in 2009. So here is a little 2013 comparison:
Expectations
- with Rspec-mocks:
expect(user).to receive(:say_hello) - with Mocha:
user.expects(:say_hello).once
Stubbing an object
- with Rspec-mocks:
user = double(name: 'John Doe') - with Mocha:
user = stub(name: 'John Doe')
Stubbing anything
- with Rspec-mocks:
User.any_instance.stub(:name).and_return('John Doe') - with Mocha:
User.any_instance.stubs(:name).returns('John Doe')
They offer the same facilities, and both can be used with or without Rspec.
So I would say choosing one over another is a matter of personal taste (and they taste quite alike).