If you have Photoshop, don't consider PDN because Photoshop IS better.
If you don't feel like paying for Photoshop then get PDN, start with that, and move on. I personally have had PDN for a year, become okay at it, and not felt a need to go to Photoshop, although I do like some of Photoshop's features.
I'm really bored now, maybe I'll take a crack at that sig.