I have a feeling that it did make a difference that the mummy routed rather than got destroyed. Even if you kill him, there is only a chance that you'll find his items, not a certainty. Some powerful magic items, and cursed items, have a greater chance of being found than run-of-the mill magic items.
One other point to keep in mind is that you don't want to employ mercenaries when you have an expectation of magical loot - the mercenary may find the item, and if he does, he'll refuse to give it up. I learned that one the hard way
