Tough call....
The FX-6350 should be able to run at a faster clock speed than the FX-8320 all else being equal. But the FX-8320 has an extra pair of integer processors (cores). I guess the difference would be if the game in question can utilize all 8 cores. If not, the FX-6350 would be better due to its ability to run at or OC to a higher speed.
Personally, since I don't play that game, I don't know how many cores Arma III supports. The forums are full of different opinions. Some say only 2. Others say up to 32. Others say only 1 is used for AI, etc. I'd suggest browsing some Arma III specific forums and getting some feedback.
Another thing you could do is open something like task manager and watch the amount of activity among the 4 cores of your Phenom II when gaming next time. See how evenly the work is spread out over the four cores. If it looks even, then go for the 8 core. (I'd probably opt for the FX-6350 myself)
Update:
I found this chart comparing thread/core usage between the FX-8350, 6300, 4300 and PhII X2 550:
http://i.imgur.com/9UZtEEt.jpg
The two processors you are looking at fall between the 8350 and 6300. As you can see, 1 core seems to do most of the work.