From P5 up, MAT poses and Material room materials essentially do the same thing, except that MAT poses are backward compatible with P4, and clutter up the incongruently named Poses folder and make people who never used Poser before v5 wonder where the heck to put them and still be able to find them when needed.

There are probably other differences I could find by hacking the files themselves, but those are the practical difference, and similarities.