Henrik,
I've only worked with the 39MS backs significantly and not the 50MS, but would bet the 50MS is as good as anything you can buy now in 4 shot mode. I now have the 80mpix aptus and it will get similar detail with hard edges like a map for instance, but not as much DR as the 39MS. I've compared scenes with soft detail like flowers and fruit and the subtle color changes are better captured with the multishot. Even if the 200MS is not a big step from the 50MS, I think it will still be a very good back. The downsides are only can you hold your subject steady and does your workflow allow for the extra time needed for multishot.
Eric