I was under the impression that Whisper White is just the new name for Ultrasmooth White. I don't believe they changed anything about the cardstock except the name.
The cheaper Georgia Pacific cardstock (from WalMart or Sam's Club) is OK until you try to use a blender pen, or any kind of watercolor. If I'm doing anything that involves blending, coloring, water, or any other manipulating, I use only SU! The cheaper cardstock just can't take it and pills up and/or breaks down.
Just my experience and two cents,
Chris