In my view, PGA is more accurate and closer to Overstreet Standards than CGC in the 4.0 - 7.0 range. In short, CGC is much too strict on midgrade books which have traditionally allowed for an accumulation of minor flaws or one or two larger flaws. Up until CGC became the "standard", FINE and VG were always fairly wide grades... but now CGC interprets them very narrowly. In some cases, it seems as if CGC goes from 7.5 straight to 4.0!
If I had the first Overstreet Grading Guide handy, I could cite irrefutable cases where midgrade books depicted would be graded at least one full grade lower today by CGC. Interestingly, with the Second Edition of the Overstreet Grading Guide -- which featured many CGC books -- the midgrade books demonstrably "got better."
I have been collecting Silver Age Fines or better as an adult for almost 20 years now. Today's CGC FINES look better than [accurately graded] FINES ever did in the past.
It's a double-edged sword... I like the idea that I can buy a CGC 6.0 - 7.0 for below Guide and the book is sharp as hell with nothing more than minor edge wear, yet, the FINE that I bought in 1988 (and EVERYONE at the time would have agreed it was a FINE) would probably get a 5.0 from CGC today.
IMO, PGA is truer to the traditional acceptance of what midgrade comics should look like.