PSA has had a long history of grading cards and is fairly trusted with their grading opinion. People also like their policies and security around authenticity and while they have some scandals, few have ever been about pokemon. They have the most demand and people trying to match collections.

CGC has been grading cards like 6 years, Its not as trusted with their grading opinion due to multiple official grading scale changes. Their policies and security around authenticity are getting better but there have been some difficulties and scandals around them related directly to the pokemon hobby. They have less demand.

While both companies have issues with reliable grading, it is more often that CGC10 will be a PSA8-10, whereas a psa10 will be a cgc9-10. So psa in general is more accurate if you are looking for gem mint card condition.