I'm the Odd Ball here.... While I do agree that PS has become the Industries Standard & that PSP also has some very good traits.
I am the guy that holds up picket signs out front of Adobe & other Monopolizing Corporate Conglomerates when I see them bumping out the little guys & forcing the smaller companies to close down.
I support the smaller software firms trying to make a name & compete. my primary Digital Design Software of Choice is Serif DP8 (Serif DrawPlus
...
DP8 is my primary Graphics tool... I can Hand sketch, scan, Convert to image format, Drag it into DP8, & utilize all the similar feature's as PS & PSP has. Including but not limited to: Layering, Animation, PFD, 3D effects, Light Box effects, Filters, Fills, Fades, Transparencies, Image Slicing (With HTML Output), DPI Control, Etc......
The Only thing it can't do that may or may not cause a problem with a potential client is the editable layered file format not being compatible with Adobe software. Then again people with Strictly PS projects are normally just piece work anyways... I much prefer the Complete project from the ground up giving me full control of the tools used to accomplish the final product.
Designers Like Artists, each have their own techniques, Styles, Preferences, & most definitely their own collection of tools. Serif Software being one of Many that are in my Design/Art tool Box....
As far as telling you which one is best between PS & PSP, I have used both... The best can only be decided by what software you are more comfortable with using & produces the best outcome (In complements to your style).
My personal choice, is Neither......