As Frederik.Sjölund writes, it's difficult to compare Sibelius and Cubase in terms of program-profiency - they are two completely different programs - one is for Music Engraving (Sibelius) and the other is for Midi Sequencing with a IMO mediocre scoring "section".
As for Music Engraving I would choose Finale - which I use almost for all my music/jobs - though I have Sibelius around for when the occation demands it. It's a matter of taste and habit I think.
I've just recently purchased the new version of Cubase - Cubase 4 - which I'm looking forward to exploring when I get the time.
But as to the choice between Cubase and Sibelius I'd say it depends on the task at hand. I haven't tried composing with Cubase yet - but maybe it'll be easier to get ideas down on "digital paper" than in Finale/Sibelius... I don't know... yet...