In a study made by the School of Mechanical and Chemical Engineering to compete in Formula SAE they ran simulations of bending stresses in SolidWorks

http://robotics.ee.uwa.edu.au/theses/2011-REV-SAE-SpaceFrame-Waterman.pdf
Page 20 Triangulation and Stressed Skins

Triangulation wins regarding strength/weight to stressed skins although they used steel panels on both sides for symmetry.
However when paneling is already needed to prevent intrusion like in the floor, firewalls and leak-proof battery boxes they used paneling.

How paneling the chassis rails compares to X-bracing regarding chassis torsional stifness?

Broad generalisations there. IT's going to depend on the exact design requirements, the quality of the modelling/sims, the materials involved, the load paths etc. But yes, outside the teams running carbon structures, most teams run space-frames,some aided by the odd panel.

Personally I'll be using a space frame setup aided by panels such as Cellite where possible (or rather, where sensible)

s