Difference Between Plug-ins & Workflows in CRM 2011

Criteria	Plug-in	Workflow
Execution	Executes immediately before or after the core operation (synchronous).Can also be queued to execute after the core operation (asynchronous).	Queued to execute after the core operation (always asynchronous).
Performance	Synchronous plug-ins can increase the platform’s response time because they are part of the main platform processing. (W3WP.exe) Asynchronous plug-ins have less impact on server response time because the code is run in a different process.(AsyncCRMServices.exe)	Less impact on server response time because the code is run in a different process. (AsyncCRMServices.exe)
Security restrictions	To register a plug-in requires a System Admin or System Customizer security role and membership in the Deployment Administrator group.	To register a workflow requires a System Admin or System Customizer security role and membership in the Deployment Administrator group.
Length of processing time	A plug-in registered for synchronous or asynchronous execution is restricted to complete its execution within a 2 minute time limit.	Works well for either short or long processes.
Process and data persistence	Plug-ins execute to completion. Plug-ins must be written to be stateless where no in-memory data is persisted.	Workflows can be paused, postponed, canceled, and resumed through SDK calls or by the user through the Web application. The state of the workflow is automatically saved before it is paused or postponed.
Impersonation	Plug-ins can perform data operations on behalf of another system user.	Workflows cannot use impersonation.
Transaction	Plugins can be registered in-transaction so they are able to roll-back the main operation if the plugin fails.	Workflows are executed post-transaction so they cannot roll back the main operation.
Image	You can register pre-images for plugin steps.	Pre-image is available only from custom workflow activities (not supported in CRM Online)