Arma 3 currently doesn't seem to care what graphics card you have and performs poorly whether you have a mid or high end card, so with that said, it doesn't matter which one you pick, you'll still get 20 - 40fps on multiplayer
From benchmarks though the nvidia cards seem to do a bit better in that game
and I've seen other arma 3 benchmarks with them ahead too. I am also an Arma 3 player (or at least I will be if it doesn't stop sucking so much ass compared to Arma 2) and this will be one of the reasons I am probably going to go with nvidia for my next card.
I'm sure either would be fine though really, also as you probably know PhysX in Arma 3 runs off the CPU and not the GPU so that isn't an issue.
Don't buy one of these cards just for Arma 3 though, I think you will be very disappointed with the performance increase you get from it. Arma 3 relies a lot on the CPU and not so much the GPU. The settings in the game that have the biggest effect on fps is view/draw distances (mainly object draw distance) which relies a lot on CPU power.Edited by smaudioz - 11/14/13 at 3:16pm