I'll direct you to the help articles for each of these methods:

Activity-List-Add

https://pdn.pega.com/sites/pdn.pega.com/files/help_v719/procomhelpmain…|Activity%2520methods|_____4

Call method

https://pdn.pega.com/sites/pdn.pega.com/files/help_v719/procomhelpmain…|Activity%2520methods|Call%2520another%2520activity|_____1

Some of the cons of using the Activity-List-Add is that using this method, Pega 7 does not check the validity of the activity you add. You have to ensure that the activity exists and that users have the necessary security to execute it. You also cannot examine these lists with the Tracer or Clipboard tool.