As shown in Xander's figures, if there is not significant elevation change, and more importantly perhaps, undulating change, the difference between a buffer that is simply 2D planar versus 3D is going to be insignificant the larger the buffer radius.
If you want to see dramatic results, pick a hill in your terrain whose 2D radius is small (maybe a couple of 100 m), then buffer that. A buffer radius over many hundreds of meters or kilometers is not going to show any significant difference.
The controlling factors
- the cumulative affect of the elevation differences from the centre to a point on the buffer radius
- the size of the buffer
the greater the first, and the smaller the second is what will produce the most pronounced result