They’re just different. One of the most significant differences is that there is no way to automatically arm/disarm SHM in the new V3 app. You have to do it manually through the app each time. In the V2 classic app, you could create a routine to arm/disarm that version of SHM Based on any event that smart things could recognize, including Geopresence, a handheld button, even voice control.
That will be important to some people, but not others.