For machining metals, esp steel, you need rigidity, so a mill arrangement, table that moves in X & Y and a rigid fixed Z column is the best solution. a rigid router style machine can mill ali if strong enough but its a compromise at best.

To improve resolution for machining metal you need to gear the motors down. a 4:1 reduction improves resolution by 4 and increases torque, necessary to overcome the inertia of a heavy moving table and increased cutting forces.