As a dark knight main, I can't quite speak for hero. I'll do my best based on what I know anyway.

Hero 1. one skill to rule them all 2. the mobbing can be painful early on. start as a dark knight if you want to avoid that 3. has a mobbing skill that's like nightshade, but it's called shout 4. more consistent dps compared to dark knight

Dark Knight 1. He has the most self buffs compared to hero, count em 6. 7 if you count your evil eye toggle 2. has a passive skill final pact that triggers when he dies making him temporarily invincible 3. only Sacrifice allows you to spam Gungnir, which has a cooldown but final pact lets you spam gungnir all over again. need to time when to die on purpose or wait the cooldown for sacrifice. 4. nice mobbing skill at 170. more nice mobbing skills with 5th job

I think both are good. it depends what you want. They equally have their pros and cons like every other class. Dark knight is good as long as you bother to utilize what he has and hero is good if you utilize what little you have to work with.

Both jump and attack to mob. they both mostly sit idle when bossing. Hero is more damage but worse mobbing and dark knight is less damage but better mobbing. It does not make them bad choices. You can pick dark knight and switch to hero later or you can pick hero and switch to dark knight later. You win any way.

Personally I love hero and dark knight equally <3