When developing new adapters create a Custom WCF Channel or use the WCF LOB SDK as a reference starting point, this will allow you to create a scalable and easy to host adapter that can be used across other .NET solutions. This level of flexibility will make the adapter more likely to be reused somewhere else besides the BizTalk world and so being more valuable than simply building it with the BizTalk Adapter framework.
If you have to decide to implement a BizTalk Adapter you will want to make it so that it can be as relevant as possible and choosing the framework you will use is an important task. The simple fact that developing adapters for WCF makes them reusable should be a compelling enough reason to decide.
You can understand the Differences Between the WCF LOB Adapter SDK and the BizTalk Server Adapter Framework here.