I would not case Attila and the Mongols in the same mould. The Mongols had a much more lasting impact because, terrible as he was in battle, the Khan's hand was light upon conquered people . . . so long as they surrendered bloodlessly and remained quiet. Moreover, the Mongols were much more purposeful in building an infrastructure of the type that underpins a successful empire: e.g., a courier system, religion, etc. The Mongols' conversion to Islam (in the West, anyway) was especially significant in that it ensured that the subjugated people of Samarkand and the Middle East remained (reasonably) happy. And in the grand scheme of things, the Mongols were the exception, rather than the rule. Just ask the Cimbri, the Huns, the Vandals, etc.