Antares Auto-tune or Melodyne?

They both work great. I think it's more about your workflow and what you want to accomplish. Check this out:http://www.soundonsound.com/sos/mar07/articles/at5vsmelodyne.htm. It's an old article but it covers the differences between them.
 
Rocky
 
 

Ok, well from that description I would say you're looking for the note splitting tool and the formant tool. Although if you need the latter, you're using pretty heavy pitch correction and it's gonna sound a little artificial anyway.
You're right, those are functions of Melodyne Editor. If you recently bought Sonar you can write to Celemony and ask for an upgrade deal, most of us here have paid 150 for the upgrade, which is a lot less than retail (or what you'd pay for Autotune, never mind iLok hassles).

I don't have much experience with Autotune honestly. I think it's better than Melodyne at full on "auto" correction but you need to sing really well for that to work convincingly. In most cases I think you'd end up wanting to tweak notes anyway and then you might as well stick with Melodyne because of ARA. Melodyne's auto correct places the center pitch of each blob 100 percent in the center of the closest note, but if there is a lot of drift inside the note that won't change. With Editor, you can correct drift.

Also, if you have an older version of Sonar Studio or Producer installed, you should have V-Vocal. Most folks feel it has been superseded by Melodyne but it does have some features that aren't included in Melodyne Essential.

I don't mean to sound discouraging, but neither option is going to help you if you really "can't sing". So I hope you were exaggerating a bit :)