[1880nonsports]

you were joking right???? Countless examples of inept grading for all the companies - see the last Heritage auction with PSA 7's that have front and back paper loss.
The major TPG's do a decent job with regard to authenticity and more easily discernable faults. You're not submitting to some kind of crime lab - for the ten bucks no-one is taking out the micrometer or neutron scale. ANYONE can render an opinion about a "grade". Such grades really only matter in terms of commerce - buying and selling the commodity.
Historically PSA cards have been shown to bring higher prices on many issues. They were first, better advertised, and developed the wildly successful registry. With the passage of time SGC has taken over the larger share of the more niche-like 19th century market and made inroads in the "T" card market. I think they do a better job, the holders look nicer, they offer excellent customer service. That said - it's a personal preference thing - you align yourself with the company whose condition/grade parameters feel "right".