I picked up the G600; ergonomically it feels nice.
I haven't tried the other options, so don't really know they're more/less comfortable.

My hands are fairly large, so the thumb & pinky grip feels good.
Not pressing the ring finger will take adjusting to, as almost every other mouse I've used was gripped by thumb + ring & pinky fingers.
I like the idea of using that as "push to talk" for Ventrilo... won't have to momentarily stop either DPS or mouse use to talk.

Even picking the mouse up with the thumb over the thumb buttons, doesn't push a thumb G-Key down.
Just need to decide which G-Key does what, and develop the muscle memory for the button.
Pushing any given button is easy enough, when you want to push it.