SimpliSafe Smart Lock or another brand?

You don't have to use a key, its available as a backup.  I would not install a smart lock without this feature.  What happens if the batteries die while you're out?  The August locks work the same way and thats what I have.  I bought mine before SS came out with theirs, but honestly I like the look of August better and would have still made that choice.  In fact, they have an even smaller lock available now.  August can also be setup for geolocation but we don't use that feature.  As far as functionality goes they are about the same.  SS alarm system ON, lock locks.  SS alarm system off, lock unlocks.  It comes down to price and looks and whether or not you think an outdoor keypad is useful.  

We use the SS app to arm and disarm so we don't even carry keys anymore.  If a battery on one dies (which hasn't happened yet) then we just use one of the other smart lock doors to gain entry.

As i understand it your existing deadbolt stays intact so wouldn't you still need a deadbolt key to gain entry? i bought the ss smart lock because i figured it would integrate with the system. i still went out and bought another deadbolt because i just bought a house and want to change the lock because i don't know who else out there might have a key to my house

Dead bolt stays.  You take off the indoor part of the lock and replace it with the new smart lock.  The inside dead bolt and outside key part of the lock stay in place.  Unlocking the door from outside can be done in a few ways.  

Use the old key

With SS lock
Optional outdoor keypad, enter your pin, door unlocks
Use the SS app to disable the alarm (which also unlocks the smart lock)
Use a key fob to disable alarm, (which also unlocks the smart lock)

With August lock
Use the August app to unlock, then walk in and disable SS alarm with indoor keypad
Use the SS app to disable the alarm (which also unlocks the smart lock)
Use a key fob to disable alarm, (which also unlocks the smart lock)
Setup geo location so lock auto unlocks when you approach it, then walk in and turn off SS alarm with indoor keypad, keyfob, or SS app

Here, this is not my video but shows you how it works.

https://www.youtube.com/watch?v=NpH4gZ0OO-U

I moved into a new house so i do not want to use the deadbolt lock that i have on there now (i don't know who else may have a key). I have bought two new deadbolts. One for the front door and one for the back door. I was going to install the ss smart lock over that. That way if someone has that old key they will not be able to use it. i was going to install the keypad outside but i might screw it in somewhere instead of the adhesive because i am concerned with someone stealing it.The key fob works really good with the alarm so i might use that too.