Technically, despite the Epic being a 5K camera, it will actually only output around a 4.25K image once debayered and most films are now mastered to 4K resolution, i.e approx. the same resolution as a good scan of a film shot on traditional 35mm. IMAX film, however is 70mm and thus has way more actual resolution than the '5K' Epic can actually put out, let alone the 4K master from which the IMAX version will be printed from.
Basically, Prometheus should look no better (or worse) than a standard movie blown up to fit on a 'proper' IMAX screen, whereas the Dark Knight Rises will look much better due to the huge amount of resolution available from 70mm film. In practise the lack of grain present and the fact that it has been kept in the digital domain throughout the production process might mean it looks a *bit* better, and it will indeed look stunning on an IMAX screen I'm sure, but just don't go along expecting the same level of 'wow' as you'll get from a 70mm IMAX print.
Hope that helps!