I also think that smacking in a cold, controlled manner is worse. It's a deliberate attack. Losing your temper in a moment of stress is understandable. Coldly deciding to hit your child and cause them pain is far worse IMO.
I don't resent the smacks I got as a child when my mother was just stressed and harassed. I do resent the ones that were a deliberate punishment - at the time I thought they were designed to humiliate, and demonstrate that my parents were bigger and stronger than me. Don't think my parents were evil or anything, it was the way people behaved then. But it's wrong, again IMO.
It used to be acceptable for husbands to beat their wives - we've moved on from there. Should make a similar move with smacking.
I'm not perfect myself and have smacked ds occasionally. But I know it's wrong and have apologised.