Hell Knight is a human class, the third class change from a Dark Avenger.
Human Fighter>Human Knight>Dark Avenger>Hell Knight
Yes, it is a dark based tank class. You get a black panther summon and some dark "magic" attacks. You are one of the stronger tanks because of your summon, but you are not a damage dealer.
Titan is an orc class. More of your in your face smash your face type of character. He can be a damage dealer, but also a tank type class, but are mainly for dealing heavy duty melee carnage.
As a subclass, it doesn't matter. Since your main class and subclass don't share skills. Sub classing is basically starting a new character with your same avatar that you can switch back and forth with.