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Define and compare, and then there is the contrast. Maybe pick a theme, as there are many, this with narrow your concern, yet try to be a little different. For example, the empires of the horse (Iranic or Hunnic) vs. those of the wheel (Neo-Assyrian) or the ship (Rome of the Principate). Also tell a story, and whatever comes of that, do not try and teach a lesson.
An excellent account on the relations between the Roman Empire and the Sasanian Persia is the Rome and Persia in late antiquity : neighbours and rivals by Beate Dignas and Engelbert Winter (Cambridge University Press, 2007) - it is quite recent and unlike most of previous studies this builds upon persian sources as far as possible. Really recommend that one to anybody! 
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