I say both. They complement each other well. If it's a matter of finance you can always pair.
Painter Essentials + Photoshop
Painter + GIMP
Photoshop Elements + Painter etc
Painter despite all it's issues is still a great piece of software. I have tried many times to do without it but what keeps me coming back is:
-The ability to realistically mix colors in the Mixer palette.
-The ability to calibrate brushes to my pressure and speed.
-A few brushes i could replicate in PS but would have a hard time doing.
Photoshop CS6 is the greatest version of Photoshop yet (PS7 fans i'm not trolling ).A few features have even been borrowed from Painter.
But at the end of the day when it comes down to simulating traditional painting on the computer Corel seems to hold a few valuable cards.
I wouldn't use Painter in a production environment though, it's way too unstable to bet thousands or millions of dollars on (there are a few of my fav artists who do however: Michael Kutsche, Ryan Church).
Now, if Adobe had implemented the Moxi Paint engine however...