I have and use RAW Power. I am currently trying out the trial of Nitro. It is still a little rough around the edges, but it has way more functionality. I particularly like that it includes so many more of the features that I miss from Aperture.
So assuming I pay for a Nitro license, is there any reason to continue using RAW Power? Are there any features in RAW Power that Nitro does not have? Are there any workflows that RAW Power handles faster — or more smoothly — than Nitro?