Smart Home Monitor is the official SmartThings feature for security notifications. It’s built into the mobile app. It can do some things that none of the other options can, in particular, it can be integrated with the subscription scout monitoring service. It’s also the only way to be able to view video clips inside the smartthings mobile app.
SmartAlarm pre-dated SHM, and was created by a community member and then officially published by SmartThings. It actually has a lot of features that SHM does not, in particular entrance/exit delays and zones. But it can’t integrate with Scout. It has remained popular with the community because of these additional features.
You can install either of these by doing the following:
One) open the official SmartThings mobile app
Two) tap the “marketplace” icon at the lower right of the screen
Three) tap “smart apps” at the top right of the screen
Four) scroll down to “safety & security” and choose that
This will display a list of available security – related smart apps. The first one will be SHM. If you scroll down a little further you will see smartalarm.