The two concepts are highly important regarding the decision to be made while choosing the approach that can be appropriate for the particular application.

Module-based approach-

The application where only simple I/O functionality is required is safe then the module-based approach is the best. The physical safety of I/Os is transferred by the module into safety Fieldbus signals and vice-versa. 

Safety protocol stack-based-  

If the safety application is more complex than exchanging the limited number of I/O bits are safe then the protocol stacks are called Safety protocol-based approach. Drive applications or safety sensors are complex as well as PLC programs that are related to safety, closely operating together with a communication entity is safe enough to exchange a vast amount of data. It is considered to be safe or the configuration of the data is done with satisfying the issues related to safety-related concerns.

Source:- ixxat.com