Darius doesn't deal 1% max HP in true damage with everything. He only gets that true damage if he keeps Conqueror refreshed if he's even running it. The Q heal he has to position himself right to get the maximum damage and heal from. He also has to get either 5 stacks on one target or a reset to gain his "true" damage potential.
However, Darius has a very very clear weakness to being kited and stunlocked. Also Darius can't get a guaranteed execute to much as people would believe. You can hover the tooltip and it will show you how much damage it can do, compared to Garen who gets a true execute if someone is low. Sure, Garen's doesn't reset, but for not having a reset, he can be tanky. Darius on the other hand has to build Damage Health and Resists to survive and keep dunking, and even then he's not guaranteed to run away with every game.
And for the E, as much as Darius gets free arm pen and a pull, Garen gets damage per spin while also refreshing the arm pen every hit after the 4th spin.
Garen gets 25% for 6+ seconds (depending on how many hits of E after the 4th) all ranks.
Darius gets 10 / 15 / 20 / 25 / 30% based on spell rank, and he always maxes this skill second.
Also, pretty sure Black Cleaver stacks with Garen's E, so there's that.