The playback of MuseScore is nice to have, but there is nothing to write home about it. The huge Sibelius library sounds more organic, but it isn’t great neither. However Sibelius opens the doors for other developers, such as EWQL and NotePerformer to take over. As result, the combined value to users is great.
At least in this aspect, the closed source software (Sibelius) is more open than the open source software (MuseScore). I certainly doubt this situation will last. However, I can not leave Sibelius unless I can have NotePerformer on MuseScore. Well, I might be too obsessed with playback, but what are scores for?