the first thing to remember is I have thrown no thumb for over 7 years with no issues, so my guess is the span. I now have 3 balls with the thumb tip style drilled into it, and I can always take out my thumb, but I was trying to take some hook off, and work more on getting the ball down the lane.
I have never had an issue with keeping the ball on the lane, but the half thumb is easier to throw a little faster, or take some of the hook off.
I feel like I am more consistent with the half thumb, and my avg jumped about 5 pins, but I have only been doing it about 6 months. The cupping is true for both styles, but I have never had a problem keeping my arm straight, and sort of cupping the ball. The ball rests solely on my hand in both cases.