Well to me there was a small difference. For example, the places the Romans conquered where never really the same, whereas the culture of the people the Mongols conquered stayed pretty much the same, though some were inriched (like Mongolia), while others were lessened (like Persia and Russia, though later leaders made up for the original burning of mosques and churches).
Also, Mongols absorbed other cultures, wheras the Romans destroyed other cultures. For example, the Romans who lived in Gaul never became Gauls, they were still Roman like, correct? Whereas the Mongols who lived in Persia became Persian like.