What is better lots or serial numbers?

Summary: Our client handles only serialized items, the buy them, stock them and then they sell them. In average they can buy any quantity around 10,000 to 1,000,000 of units per article. When the items are inside the inventory they have an average quantity of 50,000 units, they can be transfered to another inventory org or to another subinventory. The items can be sold in quantities that can vary from 100 units up to 40,000 units. Also we need this items to be able to be counted by a cycle clount and be able to make quantity adjustments. We want