I'm the other way Ryan, although I know it's because of the way these terms have been used with products I've had in the past. I used to use some software that got a new revision yearly, whatever they missed and didn't put in, became an 'Add-On'; therefore this term means incomplete to me.
Not sure if that's the case for everyone, but for me I'd rather have an upgrade than and add-on.
Marian has a point though... why not market it as a premium product of some sort.