I realize this post is a bit old, but I'm going the opposite route, and I'm new...so forgive me!
Did you end up going with the SimonXTi? I have a SimonXT (not XTi), but I am replacing it with a more robust (probably) DSC PowerSeries. I already had home automation, and have found the SimonXT's implementation of Z-Wave to be lacking.
It will work, and control lights, but it is slow. Really slow. Since all communication goes over the GSM link, it can be many seconds between sending an "on" command and getting it to work.
Instead, I have a Vera3 running locally, and that is much faster...and integrates with all sorts of smartphone apps, etc. Then it will integrate directly with the DSC alarm panel. That allows for door open notifications, etc. Then 1 touch of a button, and all the house lights go out (except for a path to the door), the alarm arms, and 30 seconds later all the doors lock.
If the system goes into alarm, I will have it immediately send a push notification (via Prowl) to my phone. I'm still debating the GSM/IP connection for monitoring.