I have and use both, Painter has a larger set of brushes that give the appearance of natural media, it also has a set of effects, lighting and texture set which is great to create a different look,  although some of this can be replicated in photoshop, it is convenient to use both rather than develop the tools in one,  I can remember earlier on when I had several different programs for purposes of exploring different options, I have settled on painter and photoshop as my tools for the things I find useful.  It is like anything else the tools are that tools and they should fit your workflow...I have one or two effects/plug ins that I use for only one of themany effects simply because they are part of my toolbox...