Hey,
When choosing software, you should think about what else you want to do with it on top of recording vocals. If all you wanted to do was record vocals, you could easily get away with using the little Sound Recorder program that comes with Windows, it will give you exactly the same results as if you recorded them with CEP or Sonar.
It's the extra features of CEP and Sonar which make them worth having, like eq, noise reduction, compression, effects, and the ability to edit and layer multiple takes in the multitrack.
You should also consider what soundcard you use to record everything. If you're recording with the soundcard that came built into your motherboard, you can expect to get only so-so results next to a higher quality card, regardless of what mic, preams or software you record into. You haven't mentioned what card you're using, forgive me if you've already taken care of this.
Take care,
Nick