The NEC does not address Bolt-in or Plug-on brekaers. The choice of breaker mounting style is strictly a design issue. that said, most specifications call for bolt-on breakers becuse that prevents contractors from installing "loadcenters" instead of "panelboards".
Note: neither the NEC nor UL consider loadcenters to be different from any other panelboard. the difference in the panels is strictly a tradition not a standard. Think 4x4 pickup versus a 1/2 ton. It comes down to personal preference and available options only.
