they were both convicted because the victims were seen as sympathetic, they themselves were unlikable, and the prosecution appealed to the jury's sense of outrage, not the facts of the case. the media certainly contributed to both convictions, by their consistent portrayals of both skakel & peterson as "guilty by reason of reputation".

  Under the first prong of Strickland, a petitioner must establish that his attorney was "ineffective." Failure to investigate adequately  is one of the most frequently raised (and probably frequently  accepted by courts) grounds for claims of ineffectiveness. But, the issue is not whether an attorney conducted a "perfect" investigation, rather it is  whether he conducted a "competent" investigation. Where things require an exercise of judgment (for example, whether to expend limited time and resources on a particular avenue of investigation or elsewhere) unless an attorney's judgment call is so lacking that no competent lawyer would have made a similar judgment call  it is generally not going to rise to the level of ineffective assistance of counsel.

  Also, as you phrased it(ineffective for not "presenting" the evidence), even greater deference is afforded the trial counsel. Meaning that if the lawyer testifies he was aware of witnesses and generally what they would have been expected to testify but chose not to present it for articulable strategic or tactical reasons it is almost never going to be found "ineffective" unless the reasons are simply incoprehensible reasons. For example, if the lawyer states I did not think the jury would find the witnesses credible in light of their personal characteristics and/or other evidence, or that the testimony would have been cumulative of other evidence from more credible and/or persuasive sources, or that he didn't use the witnesses because despite the exculpatory nature of certain testimony which could have provided he believe damaging testimny might be elicited on cross-examination, the decision not to present it is not likely to be found ineffective assistance of counsel-- even if in hindsight one might conclude it would have been helpful to the defense.

  Obviously, when a trial has resulted in the conviction of defendant of the most serious possible charges, everyone, including lawyer himself  will second guess his decisions and performance (after all it could not have been worse to have done things differently, could it?) but that does not mean he was "ineffective."