Ohgmir is the easier of the 2 builds to play. The survivability of a tower shield Ohgmir in heavy armour with pots, pipe, hp food is ridiculous. Its a straight tank build with huge sustain potential if played correctly. There is a reason everyone keeps screaming for Ohgmir nerfs. Downside is, you are slow. You arent running from anyone, and chasing them wont be fun. You stand and fight, and outlast your opponent.
Death knight sounds cool but is much harder to play well. You are way less tanky and have to rely way more on great melee skills. Necro spells are handy but require skill and timing to pull off. Small mana pool allows for only a couple of spells, so if you mess up you’ll be in a tight spot. Less hp, less damage bonus, less armour mitigation, less sustainable self healing but a more diverse playstyle. You also get to fit a few more skills into your build which can be nice.