The damage changes a lot but the percent only changes a little, since lower defense characters always have higher health. Every character has 1950 total defense and health combined (except Darkseid - 1900), so the percent is pretty close to the same over the whole cast. For example, for that combo in the video, Black Adam (combo did 494 damage) has 1000 defense and 950 hp, while Black Canary (combo did 549) has 950 defense and 1000 health. Although there's a difference of 55 damage, the combo does 52% of Black Adam's health and 54.9% of Black Canary's health which is only a difference of 2.9%.
It definitely looks like it's better to have higher defense than higher health, sadly for Scarecrow (900 defense 1050 health).
Of course, all these numbers are different at level 20, but I don't know those stats. I'm assuming it's a similar situation though.