Should the structuring of tasks in a WBS be according to vertical "features" of a product or horizontal "aspects" of a product?
For example, say I'm developing warehouse software with 3 modules:
And suppose there are two stages of development:
Would it be appropriate to structure the WBS like this?...
PROJECT Orders module Requirements Development Testing Inventory Requirements Development Testing etc...
PROJECT Requirements Orders module Inventory module Shipping module Development Orders module Inventory module Shipping module Testing etc...
Or does it not matter much?
Or should I be choosing one way or the other based on some particular aspect of the project, and if so, which aspect?