Ground Branch does not have a story mode as of yet. There´s plans for it in the future for sure. As of now (EA) you do have "Terrorist Hunt" where you as a single player go against the AI on a choosen map. You can configure the AI in various ways like the amount of "tangos" that will spawn, and other things like the reaction response times they will have (quicker, slower etc). Then you have all the other modes which are multiplayer. Ground Branch went EA yesterday after Steam gave it the go light.
You can find it at a very reasonable price below...
As for ArmA goes, well it´s ArmA
It can be real good or real bad for what it pretends to be! I personally don´t like how vehicles and airframes are simulated. The Idea is for it to be a Combined Arms doctrine, and as with DCS World they both fall short in doing so, being it because they have watered down modules or just poor implementation. In ArmA, vehicles feel like there sliding about with no ground friction, or weight what so ever, and airframes don´t feel like they have any sustain or resistence once airborne, so the overall feeling for me is a arcadish one. If your fine with that then great! In a nutshell, ArmA does very well "infantry", and DCS does very well "airframes". That said, it does not mean you can´t enjoy playing either one as a Combined Arms force, just expect to be in a tank in Arma arcade style as well as with DCS.
As mentioned, single player missions can be fun, along with other types like the use of the "Exile" mod in multiplayer (DayZ) for example, and I bet there´s more out there. With mods and dedication ArmA can be very good, but for me sometimes all the effort is just not worth it, as it can go wrong and spoil then fun for whatever "arma-reason"! I would buy it on sale...
The above is just my personal opinion!
For single player you could also roll back some years and look at "Operation Flashpoint Dragon Rising", or the "Brothers in Arms" series, both have decent single player campaigns with good AI reaction.