Inertia relates directly to an object's <em>mass</em>. It is its inherent property and expresses how much force is needed to bring the object to a certain level of acceleration. It is the sole factor.
Momentum relates proportionally to two factors: <em>mass</em> and <em>velocity</em>. As such, momentum is not an inherent property of an object but a measure dependent on it current velocity, in addition to its mass.
