Index and thumb is a lot smoother. It also allows you to make trim adjustments easier because you still have one finger on the stick. With thumbs you have to take your finger completely off the stick to make trim changes. I see more beginners trying to fly with thumbs and they are always over controlling the airplane.

Inmop I feel like I have more control using index and thumb. I have never flown with thumbs only and I couldn't imagine ever doing it.

If you put your hands out in front of you with palms up and index fingers facing each other than ya your thumbs are on top. But if your palms are facing each other like you where holding 2 cups your index and thumb are almost touching each other. It just seems more natural using the later method. Lots of good pilots fly with thumbs but that's not how I was taught. Good luck with your flying.