I think Kizaru's attack is more lethal, Luffy's more powerful.
Marco managed to recover very very swiftly from Kizaru's attack, however Kong Gatling would mess Marco more I feel. Mostly because it is shown that Garp could interrupt his regen with a single punch.
But then again that arc was full of PIS. I don't take MF seriously, not because I'm biased but I don't want to delude myself.