9663d683_Sohel_s_Blog__Code_Activity
[Options/Matrix] Object Model [WCF Service] You can use Server Object Model as the service will be hosted in SharePoint [Comments] You should not use Server object model in code activity as the workflow services can be moved to non-SharePoint server for scalability [Code Activity] 
You can use Client object model or server object model but better to user Client Object model as this gives you the flexibility to move the workflow manager (along with custom code activity) to non-SharePoint server for scalability.
Sohel's Blog
Code Activity