They do different things.  Subdivision actually generates new geometry permanently; depending on how smart the application is, that geometry can be used for all sorts of things including collision detection and physics simulation and deformation in a very predictable way.  Reyes polygon smoothing is a render-time effect and can't be used for collision, and sometimes produces some very inaccurate results when combined with things like Ambient Occlusion (up to Poser 7 at least, and I don't expect that was addressed) because the micropolygons that are created temporarily at render time aren't shaded in the way you might expect, since they aren't really "there".

My Freebies