[user104658]

Slapping is calculated - it can cause as much immediate pain as a punch but is highly unlikely to cause lasting injury (bruising, fractures, actual head injuries). Punching someone is (physically) highly risky and could end up resulting in heavy charges like ABH or even GBH - a slap (unless it's a VERY hefty slap) is only going to be considered a minor assault.

Also high risk of self-injury with a closed fist punch.