Direct Data Exchange vs. SOA

I have an inquiry that is not directly related to our mission here, but
I hope to get some good insight in response please:

Let's say we have a purchase order process between trading partners (PO
sent, Invoice received). There are (for the purposes of this inquiry) 2
possible ways to handle this process:

(1) Direct Data Exchange (create XML documents based on a common schema,
and exchange them between trading partners)

(2) SOA (have a purchase order/invoice shared service that is discovered
in a registry, etc.)

My inquiry is: What would drive an organization to use one approach or
the other, from both a business and technical standpoint? For instance,
would "critical mass of services and/or trading partners" be a driver
for SOA vs. direct data exchange?

Thanks in advance,
Joe