As fury, using Whirlwind to AOE will also increase the targets of your Bloodthirst. So, in an AoE situation(3+ mobs), you would WW before BT. More targets hit with BT = More chances to enrage.
As arms, applying colossus smash is always step 1. Use cleave (boost your WW DMG) and then Whirlwind in a 3+ mob situation. Bladestorm is a baseline arms ability now, and potentially your best AoE ability.
In Legion, play whichever you enjoy the most. Right now Arms may feel lacking damage wise, but it will probably scale and get better as you progress in your weapon.