I admit I don't do web stuff, so perhaps this is hard to do. But I think it's the ideal. Before/after comparisons are very easy to assess if you can flick between the two cases and let your eyes show you the differences. The value of having an image that's part one and part the other (and two completely separate parts!) seems a bit questionable.
(My line of work means I'm unlikely to end up a customer, so you don't have to pay attention to my opinions.)