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:

• Orders
• Inventory
• Shipping

And suppose there are two stages of development:

• Requirements
• Development
• Testing

Would it be appropriate to structure the WBS like this?...

 PROJECT
   Orders module
     Requirements
     Development
     Testing
   Inventory
     Requirements
     Development
     Testing
   etc...

Or this?...

 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?