Okay, then go to the websites of Final and Sibelius, download the demos of their full versions, play around with each, see which you prefer in terms of composition process, usability, interface, etc and then buy it and you're good to go.
If you don't even know what you'd be missing from Cubase then chances are good that you won't be missing anything at all. Either Sibelius or Finale can handle VST instruments just fine, so figure out which of the two programs you prefer and get composing. Technically, you could also consider Notion, but that one's not quite as polished yet - it's a relative newcomer to the market. Good software, just needs more tweaking. Still worth a look though.
Either way, forget about Cubase and figure out which notation software you prefer to use and then go with that one.
Good luck!
