I think that the 5.75" best plays with the strengths of the magic Compadre circuitry. Razor differentiation and discrimination, and the ability to find very tiny gold etc. The extra depth of the 8" and the ability to cover more territory sounds attractive, but one of the strengths of the Compadre is its ability to sort through a small trash filled area, evaluating each tiny signal in a carpet of trash. You can't rush that, so a larger coil is not going to help you, will hurt you.
In those trashy areas I dig even the soft signals if they are round and smooth and repeating. It is probably just a speck of foil in the grass roots, but it might be a barber dime 6" deep that sounds about the same.
I am no expert, read through the posts that some of these other guys have written about using the Compadre, that is what helped me. And it takes several outings to properly calibrate your ear to the machine, to work fast and not go crazy in a trashy small area.