VISTA 21iP vs VISTA 20P
Both the VISTA 21iP and the VISTA 20P (ADT SW3000) support up to 48 total zones. Both systems have 8 hardwired zones built unto the CPU. Both are capable of zone doubling in order to use more than the standard 8 zones without needing a zone expander. The zone doubling allows you to turn the panel 8 hardwired zones into 15.
The systems are different when it comes to IP communications for monitoring over the Internet. Whereas the VISTA 21iP has a built-in IP communicator the Vista 20P does not. The Vista 20P is capable of communicating over the Internet but you would need to add a 7847i Internet communicator.