My recommendation, if you’re willing and able to put a little work into it, is to keep the existing home security system (maybe switch to cheaper monitoring, I use GeoArm) and integrate that in with SmartThings. This will let you keep all of those existing sensors and make them smarter than ever, plus maintain the higher reliability and security of your security system. There are several threads about how to do this, depending on what kind of alarm system you have:
You can try to search the forums with your specific type to see if anyone has integrated it. My experience is with the DSC integration using an Arduino with the SmartThings shield, and it works VERY well!