It comes down to what service you want since CGC doesn't do what other companies may. In terms of grading, CGC is at least as good as CBCS, if not better (though to be dead honest, they are basically on par). In the mid grade range, books graded by either company goes for about the same. But if you are thinking of sending books you are hoping to get a 9.8 back, so much higher grades, to then sell then go for CGC. Just because they've been around longer and its easier for buyers to pay the premium that comes with a 9.8 when it's in a CGC holder.
If you have a comic that is already autographed and you really want it graded, then go with CBCS.
That's pretty much it, no use discussing other grading companies when it comes to comics.