Estimate vs Purchase Order for Work Order

What is the work order for? Who is it being issued to?

We are a flooring company, and the work order is for our subcontractors who will install/do the services.

I would issue them a Purchase Order. You can create a custom Purchase Order template to use for this subcontractor. You can even add custom fields to it. You do have to turn-on the Purchase Order option if you have not done so already.

From my understanding, you can't create a custom purchase order. That's why I ask estimate vs purchase order. Why should I use one over the other? 

It is true that Purchase Orders cannot be customized in the same way that Invoices, Sales Receipts and Estimates can in QBO. But you can use the baby gear icon and add your own custom fields to a Purchase Order, and you can turn-on custom transaction numbers. Estimates are for Customers and Purchase Orders are for vendors. If you need to present an Estimate to your Customer for this Subcontracted Service then you would create an Estimate for them.