Alarm.com SEM

Is anyone familiar with the Alarm.com System Enhancement Moduie (ADC-SEM100-LTE) which has an integrated cellular communicator and ability to integrate Honeywell panels into the Alarm.com service?  I have a Vista 21iP with a 2G onboard communicator and it's not compatible with the VISTA-GSM4G.  Easy option is to add an iGSMV4G but i'm also considering the SEM route and a switch from TC2.0 to Alarm.com.

I'm familiar with both options. The SEM is nice because it also doubles as a Z-Wave controller so that you wouldn't need to add a VAM or Tuxedo to do the automation through Honeywell. There are some limitations to the SEM as it's not a true Honeywell solution (doesn't support partitions or custom zone times for instance) but as long as you have a more basic alarm setup, it should work just fine.

The iGSMV4G solution would allow you to do IP backed up by cellular and 2 paths instead of just 1 is always a plus.
 
I think it really comes down to which interactive interface do you prefer more. If you haven't ever used Alarm.com, you should check out their site and see if you like their interface better than TC2.

Could the iGSMV4G be put inside of a standard metal locking cabinet (like the one the Vista 21iP goes in) without causing problems with the cellular reception?

If you run an external antenna to it.
 
If someone can get to your cell and tamper with it without triggering your alarm, you have bigger issues.