Thumbs.
Evolution gave us a thumb able to grip stuff. People that bench without thumbs are evolutionary closer to monkeys.
Plus having a 400lbs barbell fall 40cm down on your chest is way more stupid than using a thumb.
I doesn't matter at all in benching, especially not for the recreational bodybuilder.
Have you even tried it without thumbs. I promise you that it makes a difference, of course you feel kinda strange and insecure at first but when you got used to it you won't ever bench with thumbs around the bar again.
I just got the feeling that with thumbs my wrists aren't in a natural position to the chest anymore. I also can focus much better on the chest when the bar lies only on your ball of the hand.
I use the no thumbs grip nowadays on every exercise, also rowing and pull ups. The only exception are dumbbell exercises, where you are obviously flexible and can position your wrists in the perfect angle, and, of course, deadlifts.