I'm use them both,
and without the thumb is just dangerous with heavy weights.
Sometimes you touch your chest with the barbell, and it moves a little bit, you will lose control. I know it isn't the right form, but shit happens.
And what about the spotter, you can't expect from him doing a deadlift during if somehing happens. Cause if you lose control for whatever reason, i don't think thumbless gives you any kind of change.