  Spectral statistics of systems that undergo many--body localization
transition are studied. An analysis of the gap ratio statistics from the
perspective of inter- and intra-sample randomness allows us to pin point
differences between transitions in random and quasi-random disorder, showing
the effects due to Griffiths rare events for the former case. It is argued that
the transition for a random disorder exhibits universal features that are
identified by constructing an appropriate model of intermediate spectral
statistics which is a generalization of the family of short-range plasma
models. The model incorporates the inter- and intra-sample fluctuations and
faithfully reproduces level spacing distributions as well as number variance
during the transition from ergodic to many--body localized phase. In
particular, it grasps the critical level statistics which arise at disorder
strength for which the fluctuations are the strongest.
