Could anyone tell me the difference between the Unified Scale in the Preferences Importexport subpalette and the Scale slider found in the Tools export subpalette.
If I export a model out of ZBrush which slider will effect its scale?
Upham 
The Export>Scale slider sizes the object larger or smaller on export. The default value of 1 means that the object will export at 100% of its size (for objects created in ZBrush, that normally means 1 ZBrush unit). A value of .5 would export the object at half that size, while a value of 10 would export it at 10 times its ZBrush size.
The Unify Scale works a little differently. In past versions of ZBrush, objects that were imported into ZBrush were automatically scaled to 1 ZBrush unit along their longest dimension. When the object was exported, they were then scaled the opposite direction so that they would return to their original application and still be the proper size for that software. ZBrush 2 introduces the Unify Scale slider so that you can specify how many ZBrush units an imported object will be scaled to. By default, this value is 4 (which is meant to make it easier to sculpt extremely fine details).
The bottom line is that the Unify Scale slider is self-reversing when you import and then export a model. The Export>Scale slider, on the other hand, is a one way modification. Let’s use some examples:
Create a model in ZBrush and export it using the default settings. It will export at size A.
Create a model in ZBrush and export it using Export>Scale set to 10. It will export at size 10*A.
Create a model of size B in another application and import it with a Unify Scale value of 8. It will be 8 ZBrush units in size when it arrives in ZBrush. When you export it to your other application, it will be size B again.
Create a model of size B in another application and import it with a Unify Scale value of 8. It will be 8 ZBrush units in size when it arrives in ZBrush. Now set the Export>Scale slider to 10. When you export the model it will arrive in your other application at a size of 10*B. The Unify Scale slider restored it to its original size, which was then modified by the Export>Scale slider to determine the final size.
Hope that helps!