An (AI) Aromatase Inhibitor simply prevents the aromatase enzyme from acting with testosterone. So in a sense, it keeps the testosterone from turning into estrogen. While with a (SERM) Selective Estrogen Receptor Modulator, it will have a direct effect on your estrogen receptors rather than the estrogen itself. Basically a SERM will just prevent estrogen from being able to act on its cellular effects in the body.
The SERM will block estrogen from acting in the body if it is produced and then an AI will prevent your body from producing estrogen, two very different actions. That is why an AI is often taken on cycle to prevent estrogen production. However, with cycles that cannot have an AI as part of the regime due to its effects on the cycle, the individual would take a SERM instead because estrogen was produced on cycle and now it needs to have its effects and actions brought to a hault during the post cycle.