Anectdotally, I would say that by far the worst representation comes from  certain privately retained lawyers. Incompetent or insufferably lazy folks do not long survive as PDs because their work is subject to monitoring and supervision. Any hack with a license can claim to be a criminal defense lawyer and criminal defendants are not always the most sophisticated and discerning consumers.

   That said, it is very difficult for PD offices to retain their best because of the nature of the work and compensation. One has to be truly committed to the cause to spend years and years in a PD's office. The disparity in starting salaries while large is something many will bear for a few years to gain invaluable experience,  but down the road people often have far more need for money due to raising a family and the disparity in compensation only grows with experience.

   I think there are more cases of outright ineffective assistance (in real terms not in Strickland terms) with private counsel but, I do also believe that, with a significant number of exceptions, the most talented and experienced folks are found in private practice.

  In sum, I think PDs mostly do a very good job and relatively rarely a inexcusably bad job, but I think the private bar has more people at the extremes --good and bad.