Hi Hishori,
Both will add the lead to you program if they are not yet members.
Change program success will impact the "Success" counter. We very seldom use change program success. The very few cases we use it is to correct errors. Especially in the case when you change you member statuses settings and make a member status a success after while you already have leads in that member status.
It is strongly recommended that you use Change Program Status to keep your program member stats in line with what happens. This supposes that you have set the right programs statuses in the admin -> tag -> channel section and have chosen the appropriate channel for your program. When setting a lead to a given member status, if that status is identified as a success (with this
mark) the program success will be implicitly and automatically updated.
At any moment, if you add up number of leads that are in a success member status, the total will equal the program success count.
Hope this helps,
Greg