One the major differences for publishing schema and publishing an orchestration as a WCF Service is the Design & Coupling factor.
Publishing an orchestration requires you to get ready with the Orchestration development at first place or let your subscribers wait till you finish your orchestration development. On the other hand, publishing Schema as service will enable your subscribers get your Data Contracts and they can begin their development. Your subscribers don’t have to wait till you complete your business logic.
Another difference you will find would be – orchestrations will be tightly coupled with your clients in case if you publish orchestrations as service. Any changes in your incoming message schema causes your clients to update their service reference. Instead, if you expose a Canonical Schema as a service & develop a map between your Canonical Schema and internal Orchestration Message, any changes required in your incoming message schema will need you to update your Map between the Canonical Schema and the Incoming Message. Your subscribers don’t need to update their service references.
Moreover when we publish schema as a service, we can have more control on the Operation & Service Names generated in the WSDL.
Post your comments if there are any more differences… – Shiv