If I were you I would expand to take Susa, Ekbatana, and Phraaspa. The Zagros mountains make a very good border for your empire and the cities are wealthy enough and close enough to Perseopolis to make the expansion worhtwhile imo. You also get an excellent strategic point to launch raids against Sleukia and Babylon 2 of the stronger military and economic areas for the empire. If you wipe out their high level barracks I find that the ai never really recovers their MICs and starts to send lightly armoured units that are easily wiped out by archers and horse archers. Soon enough you will be at war with your Nomadic neighbours so trying to maintain the status quo in the east never seems worth it from what I have seen. From a role playing perspective If you have already taken Perseopolis you may as well try to establish a hegemony over the main Persian cities and take it to the Seleukids. What would Alexandros do? :)