Muradin is a really solid tank to learn on, so it wouldn’t hurt to practice him. Anub’arak is arguably one of the best tanks in the game, so unless something changes, you may see him start to be contested in terms of banning/early picks on him.
One thing that is pretty much the same on both of them is that W is your sustain/mitigation ability, and can be relatively spammed. In addition, for both heroes, E is an engage, peel, and disengage, so never use it unless you are specifically doing one of those three things. Calculate the odds of winning before engaging, especially if you are going to hard engage with E.
Remember that your main job is to set up your team to win a fight, not to chase kills or die gloriously in a 1v5. Every move you make is with the goal of giving your assassins a kill opportunity. This often means just being a road block to the enemy team’s front line, so your back line has the space to poke them down to the point where they can be finished off.
For more details on the primary functions of a tank, this was the first thread I made regarding the tank role.