Which Approach to Use

This question is almost impossible to answer since the approach used depends totally on local conditions and information.

If you want to keep the GNSS measurements totally homogenous and the information about the local map projection is available, the Classic 3D approach would be the most suitable.

For cases where there is no information regarding the ellipsoid and/or map projection and/or you want to force the GNSS measurements to tie in with local existing control, then the Onestep approach may be the most suitable.

The Twostep approach treats position and height information separately which allows for position only control points to be used as well. Compared to the onestep approach, information regarding the ellipsoid and map projection has to be known. The advantage is that this approach can be used for larger areas than the onestep.