Cubase just has a great audio engine and is a professional sequencer. Your going to get a much better mix coming from Fruity rewired into Cubase than just fruity alone. (Likewise with reason).. ... You’re getting:
-More Control...(In all avenues)
-Better Audio Engine...
-Superior Midi Implementation....

Fruity is great at what it does.. but its a more limited tool at everything else. Now that fruity 4.5 uses rewire I've started to use fruity again quite a bit as a sound generating device and leave cubase to sequencing, efx and mixing. Its hard to explain beyond that until you actually get into using cubase and you go.. "Oh ya.. Now I see!"

I taught Cubase to myself but I have a book called Cubase Reference by Wizoo thats great for when I get stuck or want to expand my horizons. There are also some instructional videos too..

As far as synths and efx:: Look at the current posts of “Favorite VST efx’s’ and “Favorite VSTi synth’s” to get all your answers.