What type of job do you have that requires you to inspect cards?
Go to the Blowout forums and research the many threads on card trimming. My rough estimate is that of the cards that have been outed, PSA has 70%, Beckett 20%, and SCG 10%. Of course that probably also roughly reflects the market share of each company.
https://www.blowoutforums.com/showthread.php?t=1290614
This thread and many others.