grip.... and is a tad easier to shoot bt with a thumb release.. I would practice with a true bt release for 3d. last thing you want is to be punching it.. then it will increase to target panic...
For me it's about anchor point and the difference in the solid feeling. A T-handle release for me allows me a more consistent anchor. It's about what feels good to you and what you find most important. Most serious 3D shooters I know all use T-handle releases, either thumb or back tension. Ron