I use both.
PSP is more 'user friendly' while PS tends to relate to the professional graphics person, except for the trend in PS7 toward the 'happy snaps digital camera' user .... not a great idea IMHO.
Baker ... PSP has and will continue to innovate, PSP6 introduce several features only picked up in PS6.
PSP8 Beta8 is looking good, if a little bit of an overkill in some areas.
Yer pays yer money and yer takes yer choice ..
One thing I have found, and this just me, myself, the PSP people have been very good over the years, from PSP4 onward, the support and service to this little hairy nosed 'bat has been excellent.
On the other hand, to use a popular term, the Adobe staff are less so and IMHO are 'up themselves'.
Both programs are fine, it depends on what features you will use and how much money you will pay.
Ergo .... PSP7 is my day to day choice but PS7 gets a run when I need to tackle summat more specialised.