The R9 280X is more powerful from memory.
What's your budget?
There are many better options if you're buying new.
Budget is pretty limited. I can get any of mentioned GPUs for the modest price. There is also second hand ASUS Radeon R9 390 8GB STRIX for the same price, but I feel something wrong with it as usually 390x is 1.5 times more expensive
That's pretty common in Arma, spikes can be pretty intense in terms of FPS.
Turn down AA or shadows/lighting, those features take way more GPU power than necessary generally.