For notation Sibelius is way better, the scoring in Cubase has not been improved for many versions, Cubase 6 scoring is the same as Cubase 5, this is an area that Steinberg should really improve.
I use Sibelius for writing and printing scores, and Cubase for actually recording it all, so until Steinberg make considerable improvements to Cubases score editor, Sibelius is the way to go, though be aware, Sibelius 7 does not work on Win XP or earlier (as in does not install at all).