I didn't realize PP was still on. I watched Grey's for years, and I watched PP occasionally, but haven't watched either in a couple of years. PP I just lost interest in, and Grey's was more a matter of timing. The episodes of PP that I watched always featured ethical dilemmas/lapses and unprofessional behavior, and I also didn't think any of the characters were very likeable. The characters on Grey's were messes in their personal lives, but usually they behaved professionally with their patients, and they were a lot more likeable.
Overall, much better writing and better character development on Grey's, IMO.