Both are way cheaper than Antares Autotune but probably not as good either. I have Voice Tweaker and it's a neat effect, but I rarely use it, as most of my stuff is intrumental. OB-Tune purports to be "based on technology created by Antares," for what that's worth...
Don't forget Melodyne 1.0. I've heard the demo but have never used it. If it works as well as the demo's profress then it might even be better that Autotune. I'll have to wait and see.