There are two radically different approches for creating 3D in vorpX. The Z-Buffer method (Z-Normal, Z-Adaptive) and Geometry 3D. Currently not all supported games have Geometry 3D.
Geometry 3D offers the most lifelike 3D effect, but costs about 50% framerate, while the Z-Buffer modes have a less natural 3D feel to them but instead guarantee you the high framerates that are needed for good (non-nauseating) head tracking.
If Z-Buffer 3D is supported, both sub-modes work. It’s purely a matter of taste what you prefer.
Especially in conjunction with the optional depth of field effect the adaptive mode partly simulates a depth cue that is missing otherwise: accomodation. Think of it as something similar to a camera autofocus. Not everybody likes that though, so there really is no right or wrong decision here, just choose what you think looks better.