Just dug out my demo version of Volcaloid 4 - with the English Luka voicepack.
I have to say that just entering notes and words, the vocal quality is distinctly better than alterego - vocaloid (without any serious editing) sounds like a voice sung without any emotion, and heavily autotuned. Alterego still sounds robotic (using the free 'bones' voicepack).
However, the lack of VST makes Vocaloid much harder to work with.
I'm going to have to spend some time with vocaloid to try and understand what the various settings / tools do, and see if it can work for me.
I'm thinking that if I can get something close in vocaloid, I can then bring it into Sonar and use Melodyne to 'tweak' it some more.....