I have an existing Napco system that I am trying to integrate with Smartthings, but I hit a snag with the 2-wire smoke detectors. I was able to integrate the contact sensors no problem using a NodeMCU 8266 controller with DH’s and a smartapp. But the smoke detectors require 12V to function and have an end of line resistor that was used and I have no way to supply 12V with the NodeMCU. I suspect it may be possible with a relay, but it seems this still may not be possible with a 12VDC 2-wire system. I may go with a new alarm board like the Vista20p, since I hate to waste the 7 smoke detectors I already have wired in.
To add one other alarm to the mix, I bought a Visonic Powermax Complete a couple of years ago and have just done the integration for that too using a Wemos D1 (easy to program and costs about £4/$5).
I picked it as its core security credentials were excellent andat a good price, so I think my integration has now given it a new lease of life for many more years. Just another thing to throw into your hat.
I think ultimately your decision should go based on the security features first and then the integration second, unless it is just for a little bit of peace of mind.
I went with Frontpoint as a 24/7 monitored system - Cellular only. I am very happy with them. I augmented my home security with Smartthings as it has so much flexibility, but sometimes unreliable. If I get an alert from Smartthings but not from Frontpoint then I know it is a false alarm and saves me from calling the police to come out for a false alarm. The two systems are not integrated and I want it that way. I also augmented Smartthings with 5 Arlo Pro cameras. I am also very happy with them. I am fairly happy with Smartthings although I wish for a little bit more reliability.