I have emailed with Pete Herzog a bunch about the cert.  the issue for most of the people i have talked with in the states is that the training & testing can be hard to come by here.  i havent taken or seen the courseware and there isnt a list of objectives except to say it covers how to use the OSSTMM to conduct an audit.  it has a practical hands on exam which speaks volumes to what it "should" be testing and any time i have spoken with a OPST/OPSA they speak highly of the course and test.

there are plenty of threads about the CPTS here in the forum so that should give you most of the background you need.

that probably does little to really answer your question though so...

if i had to choose, i would pick OPST, the training issue above is the only reason i havent.  i would love to take the training and sit for the exam though.

hope that helps