Getting caught up on who is "better" or more "reliable" is a fools errand. Both grade on a near identical scale and the former head grader of Wata is now head grader at CGC.
IMO, just submit to whichever one has the better case (in your opinion).
In terms of "trusting" the grade, never do that. lol Always look at the actual game. If it looks good, then definitely buy it. But buying a game solely off its grade is disappointment waiting to happen.