Attack speed & counterattacking

[NaughtyTiger]

If two characters are to try to attack each other, one as an action and the other as a reaction, is there anything that determines which of the two successfully delivered the blow?

I’m saying this because someone with a light and/or finesse weapon would likely be able to (given enough reaction time) be able to intercept an attack from a heavier weapon that would take a bit longer to attack with.

Since combat is happening at the same time, initiative decides who attacks first (most of the time)

If you are trying to replicate the faster attacks with light weapons, consider speed factor initiative in the DMG: cantrips, daggers, and halflings are faster than minotaurs with a maul. (for me, I run it without any dice, modifiers only at the start of the round. i find it speeds combat)